impl(bigtable): add grpc metrics exporter

[scotthart]

This is the first of several PRs to add support for exporting metrics collected by the gRPC metrics plugin. The gRPC plugin is implemented as a process wide singleton, per authority, so we have to add additional logic to ensure we only create it once, even though the application may create/destroy multiple Connections/Clients during its execution.

Additional integration testing will be added in a subsequent PR.

[gemini-code-assist]

Code Review

This pull request introduces a GrpcMetricsExporter to support OpenTelemetry metrics for Bigtable gRPC clients. The implementation includes a registry to manage exporter lifetimes and deduplication, along with necessary build configuration updates. My review highlights several critical improvements: switching from std::weak_ptr to std::shared_ptr in the registry to prevent premature plugin shutdown, optimizing performance by returning const& for latency boundaries, reducing log verbosity, and adding defensive checks for variant attribute access to ensure stability.

[codecov]

Codecov Report

❌ Patch coverage is 87.85311% with 43 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.30%. Comparing base (d799c3f) to head (b3a723f).

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/grpc_metrics_exporter.cc	70.00%	42 Missing ⚠️
...ud/bigtable/internal/grpc_metrics_exporter_test.cc	99.52%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16288      +/-   ##
==========================================
- Coverage   92.31%   92.30%   -0.01%     
==========================================
  Files        2223     2226       +3     
  Lines      208086   208440     +354     
==========================================
+ Hits       192086   192396     +310     
- Misses      16000    16044      +44     

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