fix(spanner): honor built-in metrics opt-out for gRPC metrics exporter

[rahul2393]

Fixes: #12229

setBuiltInMetricsEnabled(false) was only disabling the Spanner API tracer metrics path. During channel setup, the client could still install gRPC built-in metrics/export wiring and create the Cloud Monitoring exporter. In practice, users could
still see SpannerCloudMonitoringExporter attempts and monitoring.timeSeries.create permission failures even though startup logs reported:

  Built-in metrics enabled: false

That behavior contradicted the documented contract that setBuiltInMetricsEnabled(false) opts out of client-side metrics.

[gemini-code-assist]

Code Review

This pull request updates the enablegRPCMetrics method in SpannerOptions to check if built-in metrics are enabled before proceeding, and adds a unit test to verify that gRPC metrics are skipped when built-in metrics are disabled. The reviewer suggested renaming enablegRPCMetrics to enableGrpcMetrics to better align with Java camelCase naming conventions.