chore(gax-internal): inject diagnostic headers conditionally for integration tests

[haphungw]

Add a test-only environment variable (GOOGLE_CLOUD_TEST_EXTRA_HEADERS) to generically inject diagnostic headers during execution.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.62%. Comparing base (bf48d4f) to head (3ccfcc8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax-internal/src/observability/grpc_tracing.rs	25.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5030      +/-   ##
==========================================
- Coverage   92.66%   92.62%   -0.04%     
==========================================
  Files         226      226              
  Lines        8682     8690       +8     
==========================================
+ Hits         8045     8049       +4     
- Misses        637      641       +4     

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[westarle]

I would wait until Spanner Client Library has some public interfaces before checking in changes relating to e2e testing of trace propagation with Spanner. As long as you need to test with a fork or staged PR, the fork or PR might as well contain this change.

Another solution if we get uncomfortable with manual testing before spanner stabilizes might be to add a new feature like #[cfg(google_cloud_spanner_for_trace_test)] in the Spanner client library that (1) reexports needed spanner APIs and (2) hardcodes the header configuration in their client library.

[haphungw]

included in #5031.