## Summary of changes

Small perf tweaks for existing `AspNetCoreDiagnosticObserver`

## Reason for change

While experimenting with single span observer, noticed some (safe)
tweaks we could make to the existing implementation. There are some
other improvements we could make as well, but those are a little
riskier, so may be best to delay them.

## Implementation details

- Use `HttpContext.Items[string]` instead of
`HttpContext.Features.Get<T>` as it's a bit faster
- Make `Tracer`/`Security`/`Iast` inputs readonly, instead of using a
null-coalesce with every access
- Move check outside of `AddHeaderTagsToSpan` given common case is to
avoid the function call

## Test coverage

Covered by existing tests

## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-842

Part of a stack


- #7962
- #7963 👈
- #7964
- #7966
- #7965

## Summary of changes

Creates an optimized version of `AspNetCoreResourceNameHelper` for use
by the single-span Diagnostic observer.

## Reason for change

We have the opportunity for a variety of optimizations thanks to not
creating the MVC span, and the fact that we're .NET 6+. We could
potentially include some or all of these optimizations in the existing
observer path, but for safety/simplicity I'd rather keep it isolated for
now, and we can look at expanding it more broadly later

## Implementation details

- Use the `ValueStringBuilder` to build a resource name
- Don't pre-grab MVC route values, wait to see if we have route params
from them first
- Write directly into the builder as `ToLowerInvariant`, instead of
doing an extra ToLower() at the end

## Test coverage

Added unit tests that use the existing data for the other
implementations, to confirm it's identical. It's also covered indirectly
by the integration tests

## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-842

Part of a stack


- #7962
- #7963
- #7964
- #7966 👈
- #7965

…7965)

## Summary of changes

Adds a micro and macro benchmark for the new observer

## Reason for change

Want to verify that it has performance benefits

## Implementation details

- Added new scenario for macro
- Duplicated existing scenario for micro and run. I would like to have
had these as the same methods, but the "global" nature of diagnostic
observers makes this difficult I think.

## Test coverage

I'll manually trigger a run of the macro

## Other details

https://datadoghq.atlassian.net/browse/LANGPLAT-842

Part of a stack



- #7962
- #7963
- #7964
- #7966
- #7965 👈