impl(bigtable): add outstanding_rpcs client schema metric

[scotthart]

This PR adds the first client schema metric and the event (StubSelection) it records telemetry on.

[gemini-code-assist]

Code Review

This pull request introduces client schema metrics for Google Cloud Bigtable, specifically tracking outstanding RPCs using OpenTelemetry. It adds the client_schema_metrics implementation, headers, and unit tests, while integrating stub selection tracking into the OperationContext and Metric classes. Additionally, it refactors some loops in OperationContext to use C++17 structured bindings. Feedback on the pull request focuses on performance optimizations and code simplicity, such as returning char const* instead of std::string from static helper functions, simplifying label filtering to avoid complex abstractions, preventing unconditional fallback allocations in by_name calls, and using std::string_view in metadata processing to avoid unnecessary heap allocations.

[colinmoy]

Looks like adding the new StubSelection virtual method broke the bazel-otel-abi-2-pr check, so the ABI dump baselines need to be regenerated

[scotthart]

Looks like adding the new StubSelection virtual method broke the bazel-otel-abi-2-pr check, so the ABI dump baselines need to be regenerated

So OpenTelemetry has both an ABI 1 and an ABI 2 (we may be able to stop supporting ABI 1 some day). The issue here was not all the methods in a testing Mock were overridden in ABI 2. I will likely write another PR that relocates the OTel mocks to testing_util to help future us not run into this again.

[codecov]

Codecov Report

❌ Patch coverage is 83.50000% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.23%. Comparing base (ca5075d) to head (6b2eca2).

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/client_schema_metrics.cc	85.39%	13 Missing ⚠️
...ud/bigtable/internal/client_schema_metrics_test.cc	87.62%	12 Missing ⚠️
...oogle/cloud/bigtable/internal/operation_context.cc	54.54%	5 Missing ⚠️
google/cloud/bigtable/internal/metrics.h	0.00%	2 Missing ⚠️
...le/cloud/bigtable/internal/client_schema_metrics.h	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16360      +/-   ##
==========================================
- Coverage   92.24%   92.23%   -0.02%     
==========================================
  Files        2234     2237       +3     
  Lines      210059   210251     +192     
==========================================
+ Hits       193774   193930     +156     
- Misses      16285    16321      +36     

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