fix(metrics): correct GFE metrics extraction and enable by default

[sinhasubham]

Description

This PR resolves a critical issue where Spanner GFE (Google Front End) latency metrics were not being properly captured, and ensures these metrics are always enabled by default.

Key Changes:

• Fix Metadata Extraction Logic: Fixed a truthiness bug in MetricsInterceptor where initial_metadata() returning standard headers (e.g., content-type) masked trailing_metadata(). The server-timing header is now properly extracted from both initial and trailing metadata.
• Support for Streaming RPCs: Added _StreamingResponseWrapper and _AsyncStreamingResponseWrapper to the interceptor. This correctly defers metrics recording until the streaming iterators finish, ensuring trailing_metadata is fully populated and available before attempting extraction.
• Enable GFE Metrics by Default: Removed the gfe_enabled toggle in SpannerMetricsTracerFactory. GFE metrics capture is now always-on whenever OpenTelemetry tracing is enabled.
• Testing: Added unit tests and a new mockserver test to ensure GFE metrics are correctly published end-to-end.

[gemini-code-assist]

Code Review

This pull request introduces deferred metrics recording for streaming and async streaming RPC responses in Google Cloud Spanner by wrapping responses in specialized wrappers. It also implements GFE latency extraction from response metadata and adds corresponding tests. The review feedback highlights several critical improvements for robustness: refining streaming response detection to check for iterator methods (next and anext) rather than iterable methods to prevent incorrect wrapping of standard iterables; wrapping telemetry and metrics recording blocks in try-except blocks to ensure telemetry failures do not disrupt the main application flow; defensively validating metadata elements before unpacking to avoid unpacking errors; and properly decoding bytes metadata values before regex matching.

[parthea]

@sinhasubham, Please merge main into this branch. The presubmit failures were addressed in #17578