…pc transport" (#3400)

Reverts #3326

Context:
#3326 (comment)

s2a-gRPC is an Experimental library and shouldn't be added until it's
stabilized.

…pc transport" (#3400)

Reverts #3326

Context:
#3326 (comment)

s2a-gRPC is an Experimental library and shouldn't be added until it's
stabilized.

…pc transport" (#3400)

Reverts #3326

Context:
#3326 (comment)

s2a-gRPC is an Experimental library and shouldn't be added until it's
stabilized.

| Package | Type | Package file | Manager | Update | Change |
|---|---|---|---|---|---|
|
[com.google.api.grpc:proto-google-common-protos](https://github.com/googleapis/sdk-platform-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`2.49.0` -> `2.50.0` |
|
[com.google.cloud:google-cloud-core-http](https://github.com/googleapis/sdk-platform-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`2.48.0` -> `2.49.0` |
|
[com.google.cloud:google-cloud-spanner](https://github.com/googleapis/java-spanner)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`6.82.0` -> `6.83.0` |
|
[com.google.cloud:google-cloud-logging](https://github.com/googleapis/java-logging)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`3.20.7` -> `3.21.0` |
|
[com.google.cloud:google-cloud-datastore](https://github.com/googleapis/java-datastore)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`2.24.3` -> `2.25.1` |
|
[com.google.cloud:google-cloud-core](https://github.com/googleapis/sdk-platform-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`2.48.0` -> `2.49.0` |
| [com.google.api:gax](https://github.com/googleapis/sdk-platform-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`2.58.0` -> `2.59.0` |
|
[com.autonomousapps.dependency-analysis](https://github.com/autonomousapps/dependency-analysis-android-gradle-plugin)
| plugin | misk/gradle/libs.versions.toml | gradle | patch | `2.6.0` ->
`2.6.1` |
| [com.datadoghq:dd-trace-api](https://github.com/datadog/dd-trace-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`1.43.0` -> `1.44.1` |
| [com.datadoghq:dd-trace-ot](https://github.com/datadog/dd-trace-java)
| dependencies | misk/gradle/libs.versions.toml | gradle | minor |
`1.43.0` -> `1.44.1` |
| [software.amazon.awssdk:sdk-core](https://aws.amazon.com/sdkforjava) |
dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
|
[software.amazon.awssdk:dynamodb-enhanced](https://aws.amazon.com/sdkforjava)
| dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
| [software.amazon.awssdk:dynamodb](https://aws.amazon.com/sdkforjava) |
dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
| [software.amazon.awssdk:aws-core](https://aws.amazon.com/sdkforjava) |
dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
| [software.amazon.awssdk:bom](https://aws.amazon.com/sdkforjava) |
dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
| [software.amazon.awssdk:auth](https://aws.amazon.com/sdkforjava) |
dependencies | misk/gradle/libs.versions.toml | gradle | patch |
`2.29.32` -> `2.29.34` |
| [com.amazonaws:aws-java-sdk-sqs](https://aws.amazon.com/sdkforjava)
([source](https://github.com/aws/aws-sdk-java)) | dependencies |
misk/gradle/libs.versions.toml | gradle | patch | `1.12.779` ->
`1.12.780` |
| [com.amazonaws:aws-java-sdk-s3](https://aws.amazon.com/sdkforjava)
([source](https://github.com/aws/aws-sdk-java)) | dependencies |
misk/gradle/libs.versions.toml | gradle | patch | `1.12.779` ->
`1.12.780` |
|
[com.amazonaws:aws-java-sdk-dynamodb](https://aws.amazon.com/sdkforjava)
([source](https://github.com/aws/aws-sdk-java)) | dependencies |
misk/gradle/libs.versions.toml | gradle | patch | `1.12.779` ->
`1.12.780` |
| [com.amazonaws:aws-java-sdk-core](https://aws.amazon.com/sdkforjava)
([source](https://github.com/aws/aws-sdk-java)) | dependencies |
misk/gradle/libs.versions.toml | gradle | patch | `1.12.779` ->
`1.12.780` |

---

### Release Notes

<details>
<summary>googleapis/sdk-platform-java
(com.google.api.grpc:proto-google-common-protos)</summary>

###
[`v2.50.0`](https://github.com/googleapis/sdk-platform-java/blob/HEAD/CHANGELOG.md#2500-2024-11-14)

##### Features

- Add experimental S2A integration in client libraries grpc transport
([#&#8203;3326](googleapis/sdk-platform-java#3326))
([1138ca6](googleapis/sdk-platform-java@1138ca6))
- enable selective generation based on service config include list
([#&#8203;3323](googleapis/sdk-platform-java#3323))
([0cddadb](googleapis/sdk-platform-java@0cddadb))
- introduce `java.time` to java-core
([#&#8203;3330](googleapis/sdk-platform-java#3330))
([f202c3b](googleapis/sdk-platform-java@f202c3b))
- Update Gapic-Generator to generate libraries using `java.time` methods
([#&#8203;3321](googleapis/sdk-platform-java#3321))
([b21c9a4](googleapis/sdk-platform-java@b21c9a4))

##### Bug Fixes

- Fix flaky test
ScheduledRetryingExecutorTest.testCancelOuterFutureAfterStart
([#&#8203;3335](googleapis/sdk-platform-java#3335))
([e73740d](googleapis/sdk-platform-java@e73740d))
- httpjson callables to trace attempts (started, failed)
([#&#8203;3300](googleapis/sdk-platform-java#3300))
([15a64ee](googleapis/sdk-platform-java@15a64ee))
- instantiate GaxProperties at build time to ensure we get the protobuf
version
([#&#8203;3365](googleapis/sdk-platform-java#3365))
([bb2a3be](googleapis/sdk-platform-java@bb2a3be))
- protobuf version not always getting set in headers
([#&#8203;3322](googleapis/sdk-platform-java#3322))
([7f6e470](googleapis/sdk-platform-java@7f6e470))
- use BuildKit instead of legacy builder to build the Hermetic Build
images
([#&#8203;3338](googleapis/sdk-platform-java#3338))
([222fb45](googleapis/sdk-platform-java@222fb45))

##### Dependencies

- update google auth library dependencies to v1.30.0
([#&#8203;3367](googleapis/sdk-platform-java#3367))
([a31c682](googleapis/sdk-platform-java@a31c682))
- update grpc dependencies to v1.68.1
([#&#8203;3240](googleapis/sdk-platform-java#3240))
([c8e3941](googleapis/sdk-platform-java@c8e3941))

##### Documentation

- fix list num
([#&#8203;3356](googleapis/sdk-platform-java#3356))
([b7d6296](googleapis/sdk-platform-java@b7d6296))
- **hermetic-build:** indicate usage of Docker Buildkit in development
guide
([#&#8203;3337](googleapis/sdk-platform-java#3337))
([01e742d](googleapis/sdk-platform-java@01e742d))
- modify hermetic build docs
([#&#8203;3331](googleapis/sdk-platform-java#3331))
([25023af](googleapis/sdk-platform-java@25023af))

</details>

<details>
<summary>googleapis/java-spanner
(com.google.cloud:google-cloud-spanner)</summary>

###
[`v6.83.0`](https://github.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#6830-2024-12-13)

##### Features

- Add Metrics host for built in metrics
([#&#8203;3519](googleapis/java-spanner#3519))
([4ed455a](googleapis/java-spanner@4ed455a))
- Add opt-in for using multiplexed sessions for blind writes
([#&#8203;3540](googleapis/java-spanner#3540))
([216f53e](googleapis/java-spanner@216f53e))
- Add UUID in Spanner TypeCode enum
([41f83dc](googleapis/java-spanner@41f83dc))
- Introduce java.time variables and methods
([#&#8203;3495](googleapis/java-spanner#3495))
([8a7d533](googleapis/java-spanner@8a7d533))
- **spanner:** Support multiplexed session for Partitioned operations
([#&#8203;3231](googleapis/java-spanner#3231))
([4501a3e](googleapis/java-spanner@4501a3e))
- Support 'set local' for retry_aborts_internally
([#&#8203;3532](googleapis/java-spanner#3532))
([331942f](googleapis/java-spanner@331942f))

##### Bug Fixes

- **deps:** Update the Java code generator (gapic-generator-java) to
2.51.0
([41f83dc](googleapis/java-spanner@41f83dc))

##### Dependencies

- Update sdk platform java dependencies
([#&#8203;3549](googleapis/java-spanner#3549))
([6235f0f](googleapis/java-spanner@6235f0f))

</details>

<details>
<summary>googleapis/java-logging
(com.google.cloud:google-cloud-logging)</summary>

###
[`v3.21.0`](https://github.com/googleapis/java-logging/blob/HEAD/CHANGELOG.md#3210-2024-12-13)

##### Features

- Introduce `java.time` methods
([#&#8203;1729](googleapis/java-logging#1729))
([323eb33](googleapis/java-logging@323eb33))

##### Bug Fixes

- **deps:** Update the Java code generator (gapic-generator-java) to
2.51.0
([04d8868](googleapis/java-logging@04d8868))

##### Dependencies

- Update dependency io.opentelemetry:opentelemetry-bom to v1.45.0
([#&#8203;1638](googleapis/java-logging#1638))
([7e007d4](googleapis/java-logging@7e007d4))
- Update sdk platform java dependencies
([#&#8203;1736](googleapis/java-logging#1736))
([88b4cdf](googleapis/java-logging@88b4cdf))

</details>

<details>
<summary>googleapis/java-datastore
(com.google.cloud:google-cloud-datastore)</summary>

###
[`v2.25.1`](https://github.com/googleapis/java-datastore/blob/HEAD/CHANGELOG.md#2251-2024-12-13)

##### Bug Fixes

- **deps:** Update the Java code generator (gapic-generator-java) to
2.51.0
([106ee4d](googleapis/java-datastore@106ee4d))

##### Dependencies

- Update sdk platform java dependencies
([#&#8203;1685](googleapis/java-datastore#1685))
([4372350](googleapis/java-datastore@4372350))

###
[`v2.25.0`](https://github.com/googleapis/java-datastore/blob/HEAD/CHANGELOG.md#2250-2024-12-11)

##### Features

- Introduce `java.time` methods and variables
([#&#8203;1671](googleapis/java-datastore#1671))
([5a78a80](googleapis/java-datastore@5a78a80))

##### Dependencies

- Update dependency com.google.cloud:gapic-libraries-bom to v1.48.0
([#&#8203;1605](googleapis/java-datastore#1605))
([5c6a678](googleapis/java-datastore@5c6a678))

##### Documentation

- Update gapic upgrade installation instructions
([#&#8203;1677](googleapis/java-datastore#1677))
([b3fbfcc](googleapis/java-datastore@b3fbfcc))

</details>

<details>
<summary>autonomousapps/dependency-analysis-android-gradle-plugin
(com.autonomousapps.dependency-analysis)</summary>

###
[`v2.6.1`](https://github.com/autonomousapps/dependency-analysis-android-gradle-plugin/blob/HEAD/CHANGELOG.md#Version-261)

-   \[Fix]: `superClassName` can be null (Object has no superclass).

</details>

<details>
<summary>datadog/dd-trace-java (com.datadoghq:dd-trace-api)</summary>

###
[`v1.44.1`](https://github.com/DataDog/dd-trace-java/releases/tag/v1.44.1):
1.44.1

##### Components

##### Continuous Integration Visibility

- 🐛 Fix tracing JUnit5 tests in Maven projects with multiple forks
([#&#8203;8089](DataDog/dd-trace-java#8089) -
[@&#8203;nikita-tkachenko-datadog](https://github.com/nikita-tkachenko-datadog))

###
[`v1.44.0`](https://github.com/DataDog/dd-trace-java/releases/tag/v1.44.0):
1.44.0

##### Known Issues

> \[!WARNING]\
> This release contains a known issue that causes failures when using
Test Optimization to trace JUnit 5 tests in a Maven project where Maven
Surefire is configured with `forkCount` > 1.
> The issue is fixed in v1.44.1

##### Breaking Changes

> \[!WARNING]\
> Support for `X-Forwarded` header is dropped from default client IP
resolution.
> It can still be re-activated using the
`dd.trace.client-ip-header=x-forwarded` system property, or the
`DD_TRACE_CLIENT_IP_HEADER=x-forwarded` environment variable. See
[#&#8203;7946](DataDog/dd-trace-java#7946).

##### Components

##### Application Security Management (IAST)

- ✨ Set unexpected IAST exceptions to debug log level
([#&#8203;8044](DataDog/dd-trace-java#8044) -
[@&#8203;smola](https://github.com/smola))
- ✨ Increase IAST propagation to StringBuffer subSequence
([#&#8203;8038](DataDog/dd-trace-java#8038) -
[@&#8203;Mariovido](https://github.com/Mariovido))
- ✨ Increase IAST propagation to StringBuilder subSequence
([#&#8203;8026](DataDog/dd-trace-java#8026) -
[@&#8203;Mariovido](https://github.com/Mariovido))
- ✨ Add IAST propagation to String valueOf
([#&#8203;8013](DataDog/dd-trace-java#8013) -
[@&#8203;Mariovido](https://github.com/Mariovido))
- ✨ Increase IAST propagation to StringBuilder append
([#&#8203;8010](DataDog/dd-trace-java#8010) -
[@&#8203;Mariovido](https://github.com/Mariovido))
- ✨ Expand SSRF support in IAST to apache-httpclient-5 and
apache-httpasyncclient-4
([#&#8203;7920](DataDog/dd-trace-java#7920) -
[@&#8203;Mariovido](https://github.com/Mariovido))

##### Build & Tooling

- ✨ Generate Muzzle classes for Groovy instrumentations
([#&#8203;8004](DataDog/dd-trace-java#8004) -
[@&#8203;nikita-tkachenko-datadog](https://github.com/nikita-tkachenko-datadog))

##### Continuous Integration Visibility

- ✨ Support distributed traces in tests
([#&#8203;8078](DataDog/dd-trace-java#8078) -
[@&#8203;nikita-tkachenko-datadog](https://github.com/nikita-tkachenko-datadog))
- ✨ Implement fail-fast tests ordering for JUnit 5
([#&#8203;8055](DataDog/dd-trace-java#8055) -
[@&#8203;nikita-tkachenko-datadog](https://github.com/nikita-tkachenko-datadog))
- ✨ Mark JUnit 5 setup and teardown action spans as failed if
there is an error
([#&#8203;8033](DataDog/dd-trace-java#8033) -
[@&#8203;nikita-tkachenko-datadog](https://github.com/nikita-tkachenko-datadog))
- ✨ Add tracing of setup and teardown actions in JUnit 4
([#&#8203;8030](DataDog/dd-trace-java#8030) -
[@&#8203;daniel-mohedano](https://github.com/daniel-mohedano))

##### Crash tracking

- ✨ Improve crash tracking install logging
([#&#8203;8045](DataDog/dd-trace-java#8045) -
[@&#8203;PerfectSlayer](https://github.com/PerfectSlayer))

##### Data Streams Monitoring

- 🐛 Add Data Streams support in AWS SQS without raw message delivery
([#&#8203;8071](DataDog/dd-trace-java#8071) -
[@&#8203;piochelepiotr](https://github.com/piochelepiotr))
- ✨ Add new tag for enabled products / features to DSM
checkpoints
([#&#8203;8051](DataDog/dd-trace-java#8051) -
[@&#8203;kr-igor](https://github.com/kr-igor))
- 💡 Instrument self hosted Kafka connectors
([#&#8203;7959](DataDog/dd-trace-java#7959) -
[@&#8203;piochelepiotr](https://github.com/piochelepiotr))

##### Dynamic Instrumentation

- ✨ Add Micronaut 4 support for code origin for spans
([#&#8203;8039](DataDog/dd-trace-java#8039) -
[@&#8203;jpbempel](https://github.com/jpbempel))
- ✨ Refactor probe matching for methods
([#&#8203;8021](DataDog/dd-trace-java#8021) -
[@&#8203;jpbempel](https://github.com/jpbempel))
- ✨ Update the CodeOriginProbe fingerprint to not rely on a
stack walk
([#&#8203;8016](DataDog/dd-trace-java#8016) -
[@&#8203;evanchooly](https://github.com/evanchooly))
- ✨ Implement code origin support for grpc server entry spans
([#&#8203;7942](DataDog/dd-trace-java#7942) -
[@&#8203;evanchooly](https://github.com/evanchooly))

##### GraalVM native-image

- 🐛 Update Graal build-time instrumentation config for
TracePropagationStyle
([#&#8203;8065](DataDog/dd-trace-java#8065) -
[@&#8203;MattAlp](https://github.com/MattAlp))
- 🐛 Fix NoClassDefFoundError: Could not initialize class
DDSpanLink$EncoderHolder in Graal native-image
([#&#8203;8036](DataDog/dd-trace-java#8036) -
[@&#8203;mcculls](https://github.com/mcculls))
- 🐛🧹 Fix native-image generation of reactive applications
([#&#8203;8012](DataDog/dd-trace-java#8012) -
[@&#8203;mcculls](https://github.com/mcculls))

##### OpenTracing

- 🧹 Custom ScopeManagers are deprecated and will be removed in a
future release of dd-trace-ot
([#&#8203;8058](DataDog/dd-trace-java#8058) -
[@&#8203;mcculls](https://github.com/mcculls))

##### Tracer core

- ✨🧪 Service naming: split by jee deployment
([#&#8203;8064](DataDog/dd-trace-java#8064) -
[@&#8203;amarziali](https://github.com/amarziali))
- ✨ Exclude jboss mdb proxies from instrumenting
([#&#8203;8061](DataDog/dd-trace-java#8061) -
[@&#8203;amarziali](https://github.com/amarziali))
- ✨ Add a built-in trace interceptor for keeping traces
depending of their latency
([#&#8203;8040](DataDog/dd-trace-java#8040) -
[@&#8203;cecile75](https://github.com/cecile75))
- 💡 Introduce marker mechanism for eagerly initializing helpers
([#&#8203;8028](DataDog/dd-trace-java#8028) -
[@&#8203;mcculls](https://github.com/mcculls))
- 💡 Add JSON component
([#&#8203;7973](DataDog/dd-trace-java#7973) -
[@&#8203;PerfectSlayer](https://github.com/PerfectSlayer))
- ✨⚠️ Remove support for X-Forwarded in client IP
resolution
([#&#8203;7946](DataDog/dd-trace-java#7946) -
[@&#8203;smola](https://github.com/smola))

##### Instrumentations

##### Apache HttpComponents

- ✨ Expand SSRF support in IAST to apache-httpclient-5 and
apache-httpasyncclient-4
([#&#8203;7920](DataDog/dd-trace-java#7920) -
[@&#8203;Mariovido](https://github.com/Mariovido))

##### gRPC instrumentation

- 🐛 Use lower priorities for grpc server errors
([#&#8203;8043](DataDog/dd-trace-java#8043) -
[@&#8203;amarziali](https://github.com/amarziali))

##### JDBC instrumentation

- ✨ Add trace injection for prepared statements in Postgres
([#&#8203;7940](DataDog/dd-trace-java#7940) -
[@&#8203;nenadnoveljic](https://github.com/nenadnoveljic))

##### JMS instrumentation

- 🐛 Protect mdb from instrumenting multiple time the same event
([#&#8203;8062](DataDog/dd-trace-java#8062) -
[@&#8203;amarziali](https://github.com/amarziali))

##### Kafka instrumentation

- 💡 Instrument self hosted Kafka connectors
([#&#8203;7959](DataDog/dd-trace-java#7959) -
[@&#8203;piochelepiotr](https://github.com/piochelepiotr))

##### OpenTelemetry instrumentation

- 🐛 Support using OpenTelemetry Event API inside `@WithSpan`
annotated method
([#&#8203;8019](DataDog/dd-trace-java#8019) -
[@&#8203;mcculls](https://github.com/mcculls))

##### Reactor instrumentation

- 🐛🧹 Fix native-image generation of reactive applications
([#&#8203;8012](DataDog/dd-trace-java#8012) -
[@&#8203;mcculls](https://github.com/mcculls))

##### Spring instrumentation

- 🐛 Avoid double instrumenting lambdas on latest spring scheduling
([#&#8203;8005](DataDog/dd-trace-java#8005) -
[@&#8203;amarziali](https://github.com/amarziali))

##### All other instrumentations

- 🐛 Twilio: allow service name flattening
([#&#8203;8025](DataDog/dd-trace-java#8025) -
[@&#8203;amarziali](https://github.com/amarziali))
- ✨ Instrument Mulesoft 4.5.0+
([#&#8203;7981](DataDog/dd-trace-java#7981) -
[@&#8203;amarziali](https://github.com/amarziali))

</details>

<details>
<summary>aws/aws-sdk-java (com.amazonaws:aws-java-sdk-sqs)</summary>

###
[`v1.12.780`](https://github.com/aws/aws-sdk-java/blob/HEAD/CHANGELOG.md#112780-2024-12-11)

[Compare
Source](aws/aws-sdk-java@1.12.779...1.12.780)

#### **Amazon Simple Storage Service**

-   ### Bugfixes
- AWS SDK for Java 1.x now includes additional validation for Amazon S3
client APIs to handle scenarios where an empty string ('') is passed as
the key argument to the following operations: PutObject, DeleteObject,
ListObjects, GetObjectMetaData, ListObjectsV2, SetObjectTagging,
GetObjectTagging, SetObjectAcl, GetObjectAcl, SetObjectLegalHold,
GetObjectLegalHold, CopyObject, CopyPart, SelectObjectContent,
SetObjectRetention, GetObjectRetention, AbortMultipartUpload,
CompleteMultipartUpload, InitiateMultipartUpload, ListParts, UploadPart,
RestoreObjectV2, and RestoreObject. The SDK will validate the key
argument and throw an exception if it is an empty string, ensuring
correct and expected behavior.

</details>

---

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every weekday" in timezone Australia/Melbourne, Automerge - At any time
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GitOrigin-RevId: 69831bc62ea4d80cdcd42cef2aa9bd8eda28ae8c

…t libraries grpc transport (#3548)

**Revert #3400.**

**This PR re-introduces the S2A integration the Java Cloud SDK
(initially introduced in #3326, and temporarily reverted in #3400).**

**This PR does this by reverting #3400 with the following patches:**
- load the S2A APIs via reflection. This allows us to merge the code
while the [S2A API is still experimental in
gRPC-Java](https://github.com/grpc/grpc-java/blob/master/s2a/src/main/java/io/grpc/s2a/S2AChannelCredentials.java)
without introducing a diamond dependency conflict. Once the S2A APIs are
stable, the reflection logic can be removed and the S2A API can be used
directly (via a dependency on S2A API)
- fix NPE (#3401)
- use a different env var name for enabling the feature


**Below is the original description from #3326**

Modify the Client Libraries gRPC Channel builder to use mTLS via S2A if
the experimental environment variable is set, S2A is available (We check
this by using [SecureSessionAgent
utility](https://github.com/googleapis/google-auth-library-java/blob/main/oauth2_http/java/com/google/auth/oauth2/SecureSessionAgent.java)),
and a few more conditions (see `shouldUseS2A`).

Following https://google.aip.dev/auth/4115, Only attempt to use S2A
after DirectPath and DCA (https://google.aip.dev/auth/4114) are ruled
out as options. If conditions to use S2A are not met (env variable not
set, or S2A is not running in environment, etc (`shouldUseS2A` returns
false)), fall back to default TLS connection.

When we are creating S2A-enabled Grpc Channel Credentials, we first try
to secure the connection between the client and the S2A via MTLS, using
[MTLS-MDS](https://cloud.google.com/compute/docs/metadata/overview#https-mds)
credentials. If MTLS-MDS credentials can't be loaded, then we fallback
to a plaintext connection between the client and S2A.

The parallel go implementation : googleapis/google-api-go-client#1874
(now lives here:
https://github.com/googleapis/google-cloud-go/blob/main/auth/internal/transport/cba.go)

S2A Java client: https://github.com/grpc/grpc-java/tree/master/s2a

Resolving b/376258193 means that S2A.java is no longer experimental

…t libraries grpc transport (#3548)

**Revert #3400.**

**This PR re-introduces the S2A integration the Java Cloud SDK
(initially introduced in #3326, and temporarily reverted in #3400).**

**This PR does this by reverting #3400 with the following patches:**
- load the S2A APIs via reflection. This allows us to merge the code
while the [S2A API is still experimental in
gRPC-Java](https://github.com/grpc/grpc-java/blob/master/s2a/src/main/java/io/grpc/s2a/S2AChannelCredentials.java)
without introducing a diamond dependency conflict. Once the S2A APIs are
stable, the reflection logic can be removed and the S2A API can be used
directly (via a dependency on S2A API)
- fix NPE (#3401)
- use a different env var name for enabling the feature


**Below is the original description from #3326**

Modify the Client Libraries gRPC Channel builder to use mTLS via S2A if
the experimental environment variable is set, S2A is available (We check
this by using [SecureSessionAgent
utility](https://github.com/googleapis/google-auth-library-java/blob/main/oauth2_http/java/com/google/auth/oauth2/SecureSessionAgent.java)),
and a few more conditions (see `shouldUseS2A`).

Following https://google.aip.dev/auth/4115, Only attempt to use S2A
after DirectPath and DCA (https://google.aip.dev/auth/4114) are ruled
out as options. If conditions to use S2A are not met (env variable not
set, or S2A is not running in environment, etc (`shouldUseS2A` returns
false)), fall back to default TLS connection.

When we are creating S2A-enabled Grpc Channel Credentials, we first try
to secure the connection between the client and the S2A via MTLS, using
[MTLS-MDS](https://cloud.google.com/compute/docs/metadata/overview#https-mds)
credentials. If MTLS-MDS credentials can't be loaded, then we fallback
to a plaintext connection between the client and S2A.

The parallel go implementation : googleapis/google-api-go-client#1874
(now lives here:
https://github.com/googleapis/google-cloud-go/blob/main/auth/internal/transport/cba.go)

S2A Java client: https://github.com/grpc/grpc-java/tree/master/s2a

Resolving b/376258193 means that S2A.java is no longer experimental