test(o11y): add gRPC transport o11y integration test

[haphungw]

Add a new integration test grpc_transport_observability.rs to verify that transport-level metrics, spans, and logs are correctly emitted.

The test uses an in-memory collector to verify:

• gcp.client.attempt.duration metric is recorded.
• Transport span is created with Error status on failure.
• Actionable error log is emitted with correct trace correlation.

This test aims at increase coverage for transport-level signals, which were missed @ commit f6a03af28817c31bfb8f0f8c78fd998c5fb8c88c

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.56%. Comparing base (523e85a) to head (5b163ad).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5393      +/-   ##
==========================================
- Coverage   97.56%   97.56%   -0.01%     
==========================================
  Files         222      222              
  Lines       46616    46616              
==========================================
- Hits        45482    45480       -2     
- Misses       1134     1136       +2     

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