stats/otel: a79 scaffolding to register an async gauge metric and api to record it- part 2

[mbissa]

A79: This change introduces the API surface required to support asynchronous metrics (e.g., OpenTelemetry Observable Gauges) in gRPC-Go.

This change updates the internal MetricsRecorder interface to support registering asynchronous metric reporters. This is the second of three PRs. It establishes the contracts and wiring without adding the OpenTelemetry implementation logic. This functionality is required to support OpenTelemetry Observable Gauges, which allow components like RLS and xDS to report stateful metrics (e.g., current active requests) via callbacks.

RELEASE NOTES:

• Defining AsyncMetricReporter and AsyncMetricsRecorder interfaces.
• Adding RegisterAsyncReporter to the MetricsRecorder interface.
• Implementing fan-out registration logic in MetricsRecorderList to forward callbacks to all underlying recorders.
• Updating fake_metrics_recorder to support unit testing of async reporters.

[codecov]

Codecov Report

❌ Patch coverage is 34.48276% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.21%. Comparing base (f9d2bdb) to head (8547898).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/stats/metrics_recorder_list.go	47.61%	10 Missing and 1 partial ⚠️
internal/testutils/stats/test_metrics_recorder.go	0.00%	4 Missing ⚠️
experimental/stats/metrics.go	0.00%	2 Missing ⚠️
stats/opentelemetry/opentelemetry.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8755      +/-   ##
==========================================
- Coverage   83.35%   83.21%   -0.15%     
==========================================
  Files         418      418              
  Lines       32377    32414      +37     
==========================================
- Hits        26989    26972      -17     
- Misses       4022     4062      +40     
- Partials     1366     1380      +14     

Files with missing lines	Coverage Δ
experimental/stats/metrics.go	0.00% <0.00%> (ø)
stats/opentelemetry/opentelemetry.go	72.15% <0.00%> (-0.93%)	⬇️
internal/testutils/stats/test_metrics_recorder.go	71.62% <0.00%> (-1.99%)	⬇️
internal/stats/metrics_recorder_list.go	72.72% <47.61%> (-15.51%)	⬇️

... and 25 files with indirect coverage changes

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