ci(bigtable): add fake monitoring server and metric test that uses it

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces an Observability Collector Server within the CI infrastructure to support end-to-end integration testing of OpenTelemetry metrics for Google Cloud C++ client libraries, starting with Bigtable. The changes include the server implementation, protobuf definitions for observability verification, and integration tests. My feedback includes a recommendation to refactor duplicated code in the server implementation to improve maintainability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (40d3ab0) to head (c8590cf).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16266   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files        2214     2214           
  Lines      206505   206537   +32     
=======================================
+ Hits       190549   190579   +30     
- Misses      15956    15958    +2     

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