fix: Refine connecitivity metrics to capture RPCs with no response he…

[surbhigarg92]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @surbhigarg92, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refines the connectivity metrics collection within the Spanner client library to more accurately capture the state of RPCs, particularly those that might not provide complete server-timing header information. By centralizing the metric recording to the end of the RPC lifecycle and explicitly counting instances where GFE or AFE latency headers are absent, the changes ensure a more comprehensive understanding of network performance and potential issues. This enhancement provides better visibility into RPCs that previously might have been overlooked in metric reporting.

Highlights

• Refactored Metric Recording Logic: The process of recording server-timing header metrics (GFE and AFE latencies) has been moved from the onHeaders event to the onClose event of the gRPC call. This ensures that metrics are processed after the entire RPC lifecycle, including potential errors or missing headers.
• Explicitly Capture Missing Headers: The system now explicitly records a 'missing header' count for GFE and AFE latencies when the corresponding latency values are not present in the server-timing header. This addresses the original problem of not capturing RPCs with no response headers.
• Streamlined Metric API: The BuiltInMetricsTracer and CompositeTracer classes have been updated to use a single, consolidated recordServerTimingHeaderMetrics method, simplifying the interface for recording GFE and AFE latencies along with direct path and AFE enabled flags.
• Improved Test Coverage: New assertions have been added to the unit tests to verify that GFE_CONNECTIVITY_ERROR_NAME and AFE_CONNECTIVITY_ERROR_NAME metrics are correctly recorded when server-timing headers are missing or when there's no network connection.

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[gemini-code-assist]

Code Review

This pull request refactors the connectivity metrics recording to correctly capture RPCs that fail before receiving a response header. The logic is moved from onHeaders to onClose, which is a significant correctness improvement. The changes also simplify the tracer classes by consolidating metric recording methods.

My review includes a few suggestions to improve code style and maintainability, such as using specific imports, adjusting variable scope, and adhering to naming conventions. Overall, this is a solid improvement.