feat(storage): export gRPC channel creation latency metric by transport type

[kalragauri]

This PR adds storage.googleapis.com/internal/client/channel_creation_latency: an OpenTelemetry histogram, in seconds, of the elapsed time from gRPC stub creation (i.e. client construction) until the first gRPC channel in the pool becomes ready, labeled by transport_type (CloudPath, DirectPath, or DirectPathInterconnect).

Key Changes

• channel_creation_latency histogram & callback seam (channel_metrics.{h,cc}, channel_telemetry.{h,cc}, storage_stub_factory.cc):
  • Extends StartChannelTelemetry() with a ChannelReadyCallback, invoked only when the first channel becomes ready (status.ok()). Failed, timed-out, or abandoned connection attempts are not recorded, so the timeout value does not skew the histogram.
  • MakeChannelMetricsCallback() creates the internal/client/channel_creation_latency double histogram once per stub creation under the gl-cpp instrumentation scope, and records the elapsed time with the transport_type label.
  • Returns an empty callback when built without GOOGLE_CLOUD_CPP_STORAGE_WITH_OTEL_METRICS, or when no meter provider was registered for the client's authority (metrics disabled via EnableGrpcMetricsOption, or no monitoring project available).
• Internal metric routing & latency histogram view (metrics_exporter_options.{h,cc}, metrics_meter_provider.{h,cc}):
  • MetricsExporterOptions() routes instruments prefixed with internal/client/ to storage.googleapis.com/internal/client/<name>, while gRPC's built-in instruments stay under storage.googleapis.com/client/.
  • MakeGrpcMeterProvider() registers a latency histogram view (MakeLatencyHistogramBoundaries(), 2ms buckets up to 100ms, then widening up to 5 minutes) for (gl-cpp, internal/client/channel_creation_latency).
• Shared, thread-safe MeterProvider lookup (metrics_exporter_impl.{h,cc}):
  • ExporterRegistry now retains the MeterProvider built by EnableGrpcMetricsImpl() per authority and exposes FindMeterProvider(authority). Provider creation and global gRPC plugin registration happen under the registry mutex, so concurrent clients for the same authority never observe a partially initialized entry.
  • Reusing the existing provider means channel_creation_latency shares one periodic exporter with gRPC's metrics and inherits the customer's EnableGrpcMetricsOption opt-out, export period/timeout, and monitoring project, without a second exporter.

[gemini-code-assist]

Code Review

This pull request introduces gRPC channel creation latency metrics tracking using OpenTelemetry. It adds a new channel_creation_latency histogram under the internal storage.googleapis.com/internal/client/ namespace, sharing the exporter and meter provider with gRPC's own metrics. The changes integrate a ChannelReadyCallback into the channel telemetry lifecycle and stub factory, supported by comprehensive unit tests. The review feedback correctly identifies two style guide violations in channel_metrics.cc where absl::string_view was used instead of std::string_view.

[codecov]

Codecov Report

❌ Patch coverage is 96.97987% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.36%. Comparing base (b60d64c) to head (b03e1b3).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...loud/storage/internal/grpc/channel_metrics_test.cc	98.00%	3 Missing ⚠️
...ud/storage/internal/grpc/channel_telemetry_test.cc	84.21%	3 Missing ⚠️
...oud/storage/internal/grpc/metrics_exporter_impl.cc	94.73%	2 Missing ⚠️
...age/internal/grpc/metrics_exporter_options_test.cc	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16538      +/-   ##
==========================================
+ Coverage   92.35%   92.36%   +0.01%     
==========================================
  Files        2263     2265       +2     
  Lines      218651   219046     +395     
==========================================
+ Hits       201933   202329     +396     
+ Misses      16718    16717       -1     

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[v-pratap]

The channel_creation_latency histogram integration, internal/client/ metric routing, and thread-safe ExporterRegistry lookup look very clean. Left a few comments on channel_metrics.cc and channel_metrics_test.cc around guarding against a null histogram from CreateDoubleHistogram(), honoring per-client EnableGrpcMetricsOption(false) when a provider was already registered for the same authority, and asserting the recorded latency value in seconds.