feat(storage): add OpenTelemetryMetricsOption and env var support

[bajajneha27]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces the OpenTelemetryMetricsOption to enable metrics collection with OpenTelemetry in the Google Cloud C++ SDK. It adds support for configuring this option via the GOOGLE_CLOUD_CPP_OPENTELEMETRY_METRICS environment variable, propagates it through common and authentication options, integrates it into the storage client options, and includes corresponding unit tests to verify the behavior. There are no review comments, so I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.29%. Comparing base (6f13a6b) to head (7027a4d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16301   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files        2226     2226           
  Lines      208573   208644   +71     
=======================================
+ Hits       192504   192578   +74     
+ Misses      16069    16066    -3     

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