A50: Exclude locally-initiated client cancellations from Call Counter (#12923)

[AgraVator]

Summary

Clarifies gRFC A50 (A50-xds-outlier-detection.md) to explicitly define that locally-initiated client-side cancellations MUST be excluded from the Outlier Detection Call Counter (successCount and failureCount), following the cross-language discussion consensus.

Key Clarifications

1. Picker Counting (~line 140): Excludes locally-initiated client-side cancellations (hedging cancellations, application CANCELLED, and DEADLINE_EXCEEDED) from counter increments on call completion.
2. Call Counter (~line 173): Clarifies that active buckets track successCount and failureCount, omitting locally cancelled calls.
3. Rationale (~line 285): Explains that locally-initiated client-side cancellations (including DEADLINE_EXCEEDED, which is treated as a client-side cancellation from first principles and for implementation simplicity) are excluded from outlier detection counting to align with Envoy's resetStream() behavior and prevent false-positive ejections during hedging.

Discrepancy with gRPC-Java PR #12923

• Discussion Consensus (Source of Truth): In the discussion thread, Mark Roth concluded that DEADLINE_EXCEEDED should be treated as a cancellation and excluded from outlier detection counting along with hedging cancellations and application CANCELLED.
• Implementation Discrepancy: gRPC-Java PR grpc/grpc-java#12923 currently only checks status.getCode() == Status.Code.CANCELLED in AbstractClientStream.java (line 460). Because DEADLINE_EXCEEDED has its own distinct status code in Java (Status.Code.DEADLINE_EXCEEDED), PR #12923 does not yet exclude DEADLINE_EXCEEDED from Call Counters.
• Action Required: PR #12923 should be updated to also trigger clientCancelled(status) when status.getCode() == Status.Code.DEADLINE_EXCEEDED to match this gRFC and the cross-language consensus.

Related Work

• gRPC-Java PR #12923: https://github.com/grpc/grpc-java/pull/12923/changes
• Envoy Outlier Detection: https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/upstream/outlier