feat(gax): emit T4 attempt spans

[shivanee-p]

Emits OpenTelemetry tracing spans per-attempt transport spans (SpanKind.CLIENT) directly from google-gax for both gRPC and HTTP/REST fallback calls

Key Changes:

• Added traceAttempt() and AttemptTraceContext to wrap each individual transport attempt (<apiName>/<methodName>, SpanKind.CLIENT), parented to the enclosing T3 client call span (<ClientName>.<MethodName>, SpanKind.INTERNAL)
• Populated T4 span attributes: gcp.client.service/rpc.system, rpc.method/http.request.method, url.domain, server.address, server.port, gcp.grpc.resend_count/http.request.resend_count, error.type, span error status, and exception events on attempt failures
• Propagated servicePath / apiEndpoint and port from GrpcClient options into constructSettings otherArgs so static trace context resolves custom endpoints and ports
• Added unit tests covering traceAttempt, T3/T4 parent-child hierarchies across unary, retries, and streaming calls, HTTP verb resolution, and transport-specific status code attributes

[gemini-code-assist]

Code Review

This pull request implements OpenTelemetry trace context propagation into gRPC and HTTP/REST metadata within the gax package, introducing an isMetadata utility and a comprehensive test suite. The review feedback points out that plain object metadata should be cloned before injecting the active span context to prevent unintended mutations and trace context leakage.

[shivanee-p]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces Tier 4 (T4) per-attempt tracing spans (OpenTelemetry CLIENT spans) parented under the T3 client request spans in google-gax. It adds traceAttempt in TracerHelper.ts to manage individual RPC transport attempts, captures service path and port configurations in both gRPC and fallback clients, and updates unit tests to verify the new span relationships and attributes. Feedback on the changes highlights a critical issue in createApiCall.ts where mutating the outer func parameter inside invokeCall causes a memory leak and call stack growth across multiple API calls; a fix is suggested to wrap currentApiCaller instead.