GeezerVison

Problem / Context

To capture transient wire failures and retries across synchronous and asynchronous gRPC transports without blocking in-flight grpc.Future or grpc.aio.UnaryUnaryCall objects, google.api_core._observability needs gRPC client interceptors that inspect completed RPC attempts and emit structured DEBUG logs correlated with the active Tier 4 OpenTelemetry span.

Bug: b/479827845

Solution

• Add _GrpcAttemptLoggingInterceptor and _AsyncGrpcAttemptLoggingInterceptor in google/api_core/_observability.py to log failed unary-unary gRPC wire attempts at DEBUG level via log_transport_attempt.
• Attach post-dispatch status and exception inspection via add_done_callback when supported on the returned gRPC response/call object so grpc.Future and grpc.aio.UnaryUnaryCall execution remains non-blocking.
• Update get_otel_interceptor and get_otel_async_interceptor to sandwich the attempt logging interceptor between the outer OpenTelemetry tracing interceptor (so the Tier 4 span is active for trace correlation) and the inner _SuppressingClientInterceptor / _AsyncSuppressingClientInterceptor.
• Add unit tests in tests/unit/test_observability.py covering sync/async response callbacks, direct exceptions, fail-open inspection errors, and interceptor ordering.

Verification

Relying on GitHub Actions CI for full matrix verification, alongside local unit test (unit-3.14) and lint verification.