refactor(bigtable): decouple metric cloning from OperationContext

[scotthart]

This PR decouples ResourceLabels and DataLabels from the OperationContext constructor. This increases the cohesiveness and genericity of the OperationContext class. This will be useful for adding Client Schema metrics to Bigtable and required for metrics support across other libraries.

[gemini-code-assist]

Code Review

This pull request refactors metric cloning by moving the cloning logic out of OperationContext and introducing a standalone CloneMetrics helper function. This simplifies the OperationContext constructor to directly accept a vector of already-cloned metrics, and allows the removal of the CloningMetric test helper class across multiple test files. Feedback on the changes suggests replacing the assert in CloneMetrics with an explicit nullptr check to prevent potential null pointer dereferences in release builds where assertions are disabled.

[gemini-code-assist]

In release builds, assert is compiled out, which means a null pointer in metrics would lead to a null pointer dereference and undefined behavior (crash) when calling m->clone(...). To ensure robustness and adhere to defensive programming practices, we should explicitly check for nullptr before dereferencing.

  for (auto const& m : metrics) {
    if (m == nullptr) continue;
    v.emplace_back(m->clone(resource_labels, data_labels));
  }

References

1. Prefer defensive code, such as explicit ok() checks, even if they seem redundant based on the current implementation of a framework, as the framework's contract may change in the future.

[scotthart]

Done.

[dbolduc]

I agree with the bot

[codecov]

Codecov Report

❌ Patch coverage is 99.43503% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (ac64c89) to head (6eef5c6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/bigtable/internal/metrics.cc	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16355      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.01%     
==========================================
  Files        2227     2227              
  Lines      209594   209531      -63     
==========================================
- Hits       193335   193268      -67     
- Misses      16259    16263       +4     

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