chore(spanner): support built-in metrics for ResultSet streaming RPCs

[olavloite]

Adds OpenTelemetry built-in metrics instrumentation for streaming operations (ExecuteStreamingSql and StreamingRead) in Cloud Spanner ResultSet.

• Records attempt_count, attempt_latencies, gfe_latencies, and afe_latencies across all initial attempts and stream restarts.
• Records end-to-end operation_count and operation_latencies spanning the entire query lifecycle until stream completion or permanent failure.
• Preserves attempt response headers across background stream trailer drains (seen_last) and early drops.
• Standardizes metric method attribute formatting to "Spanner.<MethodName>" (Spanner.ExecuteStreamingSql, Spanner.StreamingRead) matching Java and Go reference clients.
• Parses single and multiple server-timing response headers for GFE and AFE timings.

[gemini-code-assist]

Code Review

This pull request introduces attempt and operation metrics tracking for Spanner streaming queries and reads, including parsing server-timing headers for GFE/AFE latencies and normalising method names. The review feedback highlights several critical gaps in error handling and metrics accuracy: stream initialization failures are currently not recorded (and incorrectly reported as successful upon drop), stale headers from previous attempts are not cleared during retries, and complete connection failures prior to ResultSet creation are missed entirely by the metrics recorder.

[codecov]

Codecov Report

❌ Patch coverage is 97.20972% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.15%. Comparing base (3fb8e6d) to head (3740e16).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
src/spanner/src/observability/metrics.rs	94.69%	12 Missing ⚠️
src/spanner/src/result_set.rs	98.77%	10 Missing ⚠️
src/spanner/src/batch_read_only_transaction.rs	82.50%	7 Missing ⚠️
src/spanner/src/server_streaming/stream.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6302      +/-   ##
==========================================
- Coverage   96.26%   96.15%   -0.11%     
==========================================
  Files         282      282              
  Lines       72762    73739     +977     
==========================================
+ Hits        70044    70905     +861     
- Misses       2718     2834     +116     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements built-in metrics collection for Spanner streaming operations, tracking attempt and operation counts, latencies, and GFE/AFE latencies parsed from response headers. It updates ResultSet to record these metrics during stream initialization, execution, retries, and upon being dropped, and adds comprehensive unit and integration tests to verify the metrics recording behavior. The review feedback suggests extending the stream auto-trait assertions to explicitly verify the Sync trait, ensuring compatibility with thread-safe types.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements attempt and operation metrics tracking for Spanner streaming RPCs (ExecuteStreamingSql and StreamingRead), including parsing GFE/AFE latencies from response headers and recording metrics on completion or failure. While the implementation is well-tested, the review feedback highlights a critical blind spot: initial stream creation failures (when the initial send().await fails before a ResultSet is instantiated) are currently not recorded in the metrics. To ensure complete observability, these initial failure paths in both batch_read_only_transaction.rs and the retry macro in read_only_transaction.rs should be updated to record attempt and operation metrics.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces comprehensive observability improvements to the Spanner client, specifically by adding support for recording GFE and AFE latencies via server-timing headers and normalizing gRPC method names for metrics. Key changes include updating ResultSet to track attempt and operation durations, refactoring retry logic to capture these metrics, and adding extensive test coverage for the new observability features. The reviewer provided valuable feedback on reducing code duplication by extracting shared logic into helper functions for both the streaming RPC execution and error handling, as well as suggestions for simplifying match arm logic in the metrics module.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements built-in metrics collection for Spanner streaming operations, including ExecuteStreamingSql, StreamingRead, and BatchWrite. It updates ResultSet and the server streaming builders to capture response headers (specifically parsing 'server-timing' for GFE and AFE latencies) and track attempt and operation durations. Additionally, it introduces gRPC method name normalization, disables metrics for plaintext endpoints, and adds comprehensive unit and integration tests to verify metrics recording under various success, retry, and failure scenarios. I have no further feedback to provide as the implementation is robust and well-tested.