impl(bigtable): enable outstanding_rpcs metric and StubSelection event

[scotthart]

This PR wires the new outstanding_rpcs metric and associated StubSelection event into OperationContext, OperationContextFactory, the stub selection decorator, and DynamicChannelPool, enabling it to be collected and exported.

[gemini-code-assist]

Code Review

This pull request updates the Bigtable client to track and export client-side metrics, specifically propagating OperationContext through the BigtableRandomTwoLeastUsed decorator and updating DynamicChannelPool to expose its TransportType. It also enhances MetricsOperationContextFactory to populate ClientResourceLabels for client-schema metrics. A critical issue was identified in operation_context_factory.cc where the monitored resource type for client-side metrics is incorrectly set to "bigtable_table" instead of "bigtable_client", which would cause Google Cloud Monitoring to reject these metrics due to label mismatches.

[gemini-code-assist]

For client-side metrics (such as outstanding_rpcs), the monitored resource type in Google Cloud Monitoring should be "bigtable_client" rather than kResourceType (which is "bigtable_table"). Using kResourceType will cause the metrics to be rejected by the Monitoring service because the client-specific labels (like client_name, uuid, etc.) do not match the schema of the "bigtable_table" monitored resource.

        google::api::MonitoredResource resource;
        resource.set_type("bigtable_client");

[scotthart]

Line 249: auto constexpr kResourceType = "bigtable_client_raw";

If anything, these lines could be factored out to the enclosing scope.

[codecov]

Codecov Report

❌ Patch coverage is 91.84953% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.25%. Comparing base (31a0d9f) to head (853510b).

Files with missing lines	Patch %	Lines
...oud/bigtable/internal/operation_context_factory.cc	86.88%	16 Missing ⚠️
...igtable/internal/operation_context_factory_test.cc	92.24%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16362      +/-   ##
==========================================
+ Coverage   92.23%   92.25%   +0.02%     
==========================================
  Files        2237     2237              
  Lines      210251   210434     +183     
==========================================
+ Hits       193928   194144     +216     
+ Misses      16323    16290      -33     

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[colinmoy]

There appears to be missing test coverage in the dynamic_resource_fn block in operation_context_factory.cc