fix(bigtable): fix build when OTel metrics disabled

[colinmoy]

Define StubSelectionParams and the no-op OperationContext::StubSelection when GOOGLE_CLOUD_CPP_BIGTABLE_WITH_OTEL_METRICS is not defined.

[gemini-code-assist]

Code Review

This pull request refactors the metrics header to expose StubSelectionParams unconditionally and adds a default no-op implementation of OperationContext::StubSelection. To prevent duplicate definition errors at link time when OpenTelemetry metrics are enabled, the no-op implementation of StubSelection should be wrapped in a conditional compilation block.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.36%. Comparing base (fb87c70) to head (5a05dc1).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16519      +/-   ##
==========================================
- Coverage   92.38%   92.36%   -0.02%     
==========================================
  Files        2262     2262              
  Lines      217100   217100              
==========================================
- Hits       200557   200525      -32     
- Misses      16543    16575      +32     

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