Move the logic to read GCE_METADATA_TIMEOUT env variable so we can better control it in app_identity component.

Related internal change is 607513254.

ludoch changed the title Move the logic to read GCE_METADATA_TIMEOUT. Update _metadata.py

clundin25 changed the title Update _metadata.py feat: Use float instead of int for GCE_METADATA_TIMEOUT

* chore(python): update dependencies in /.kokoro (googleapis#1499)

Source-Link: googleapis/synthtool@db94845
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:a8a80fc6456e433df53fc2a0d72ca0345db0ddefb409f1b75b118dfd1babd952

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

* Add a test with new _metadata.py changes

* Add tests for  metadata changes.

* chore(python): bump idna from 3.4 to 3.7 in .kokoro (googleapis#1517)

Source-Link: googleapis/synthtool@d50980e
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:5a4c19d17e597b92d786e569be101e636c9c2817731f80a5adec56b2aa8fe070

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

* Update environment_vars.py

---------

Co-authored-by: gcf-owl-bot[bot] <78513119+gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Currently, `google.auth.default()` always attempts to authenticate using
the Google Compute Engine (GCE) metadata service as a fallback. There is
no easy way to prevent this behavior, leading to unnecessary delays and
unhelpful exceptions, particularly in cases where GCE-based
authentication is pointless or undesired.

This pull request introduces a `NO_GCE_CHECK` environment variable,
allowing users to explicitly skip GCE metadata service authentication.
This implementation mirrors `NO_GCE_CHECK` in
[googleapis/google-auth-library-java](https://github.com/googleapis/google-auth-library-java/blob/f154edb3d8503d29f0020b6904dfa40e034ded93/README.md?plain=1#L106-L119):

> _The following are searched (in order) to find the Application Default
Credentials:_
>
> [...]
>
> 5. Google Compute Engine built-in credentials
> - Skip this check by setting the environment variable
`NO_GCE_CHECK=true`

> [!NOTE]
> Other clouds also provide similar environment variables, such as
`AWS_EC2_METADATA_DISABLED` on AWS.

This change is also tangentially related to the following pull requests:
- #1488
- #1481

---------

Co-authored-by: Anthonios Partheniou <partheniou@google.com>
Co-authored-by: Chalmer Lowe <chalmerlowe@google.com>

Currently, `google.auth.default()` always attempts to authenticate using
the Google Compute Engine (GCE) metadata service as a fallback. There is
no easy way to prevent this behavior, leading to unnecessary delays and
unhelpful exceptions, particularly in cases where GCE-based
authentication is pointless or undesired.

This pull request introduces a `NO_GCE_CHECK` environment variable,
allowing users to explicitly skip GCE metadata service authentication.
This implementation mirrors `NO_GCE_CHECK` in
[googleapis/google-auth-library-java](https://github.com/googleapis/google-auth-library-java/blob/f154edb3d8503d29f0020b6904dfa40e034ded93/README.md?plain=1#L106-L119):

> _The following are searched (in order) to find the Application Default
Credentials:_
>
> [...]
>
> 5. Google Compute Engine built-in credentials
> - Skip this check by setting the environment variable
`NO_GCE_CHECK=true`

> [!NOTE]
> Other clouds also provide similar environment variables, such as
`AWS_EC2_METADATA_DISABLED` on AWS.

This change is also tangentially related to the following pull requests:
- googleapis/google-auth-library-python#1488
- googleapis/google-auth-library-python#1481

---------

Co-authored-by: Anthonios Partheniou <partheniou@google.com>
Co-authored-by: Chalmer Lowe <chalmerlowe@google.com>