refactor(bigtable): move common otel mocks to testing_util

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request refactors OpenTelemetry metrics testing utilities by extracting duplicated mock classes (MockHistogram, MockCounter, MockMeter, MockMeterProvider) and the MakeAttributesMap helper function from Bigtable internal tests into shared testing utilities under google/cloud/testing_util. Feedback on the new headers suggests correcting the included OpenTelemetry headers to directly include the required KeyValueIterable and nostd types instead of relying on transitive or incorrect includes.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (31a0d9f) to head (b8b8b2c).

Files with missing lines	Patch %	Lines
...le/cloud/testing_util/mock_opentelemetry_metrics.h	26.31%	14 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16361   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files        2237     2239    +2     
  Lines      210251   210227   -24     
=======================================
- Hits       193928   193913   -15     
+ Misses      16323    16314    -9     

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