fix(spanner): scope server-timing metrics per call and guard interceptor lifecycle callbacks

[olavloite]

• Scope gfeLatency and afeLatency to per-call listener instances in HeaderInterceptor to eliminate data races and cross-RPC telemetry pollution.
• Guard onHeaders and onClose in HeaderInterceptor with try...finally to guarantee downstream callback propagation (preventing hung futures) and RequestIdTargetTracker cleanup.
• Catch Throwable in SpannerErrorInterceptor.onClose to prevent unexpected metadata parsing errors from escaping into gRPC transport threads.

The above changes should guarantee that all interceptors in the entire chain of interceptors are always executed, and that no exceptions escape to the gRPC thread executing them.

[gemini-code-assist]

Code Review

This pull request refactors HeaderInterceptor to make it thread-safe by moving latency state variables from the class level to the call listener level. It also enhances robustness in both HeaderInterceptor and SpannerErrorInterceptor by wrapping header and close processing in try-catch-finally blocks, ensuring downstream gRPC notifications are always executed even if unexpected exceptions occur. Additionally, comprehensive unit tests are introduced for both interceptors to verify their behavior and error handling. I have no feedback to provide as there are no review comments.