impl(bigtable): update observability integration test to test all metric types

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request updates the observability integration tests to support both static and dynamic channel pools, redirects Cloud Monitoring metric exports to a local OpenTelemetry collector, and introduces a new test case VerifyOutstandingRpcsMetric for dynamic pools. It also refactors the metric verification logic using custom GTest matchers for improved readability. The feedback suggests replacing absl::string_view with std::string_view in two locations to comply with the repository style guide.

[codecov]

Codecov Report

❌ Patch coverage is 3.70370% with 104 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (31a0d9f) to head (ae2bf11).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/bigtable/tests/observability_integration_test.cc	3.70%	104 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16363      +/-   ##
==========================================
- Coverage   92.23%   92.23%   -0.01%     
==========================================
  Files        2237     2237              
  Lines      210251   210269      +18     
==========================================
+ Hits       193928   193934       +6     
- Misses      16323    16335      +12     

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