fix: preserve channel configurator for grpc-gcp and add opt-out for gcp OTel metrics

[rahul2393]

• Preserve the existing channel configurator when enabling grpc‑gcp so gRPC metrics instrumentation is not dropped.
• Add a toggle to disable grpc‑gcp OpenTelemetry meter injection via SpannerOptions and
  SPANNER_DISABLE_GRPC_GCP_OTEL_METRICS.
• Add unit tests covering configurator preservation and the new opt‑out behavior.

[gemini-code-assist]

Summary of Changes

Hello @rahul2393, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses two key areas related to gRPC and metrics in the Spanner client library. It ensures that custom channel configurations are correctly maintained when grpc-gcp extensions are activated, preventing loss of existing gRPC instrumentation. Additionally, it provides users with explicit control over the automatic injection of OpenTelemetry metrics for grpc-gcp, allowing them to disable this feature if desired. These changes improve the flexibility and robustness of the client's gRPC integration and metrics handling.

Highlights

• Channel Configurator Preservation: The existing ChannelConfigurator is now preserved when grpc-gcp is enabled, ensuring that other gRPC instrumentation (like metrics) is not inadvertently dropped.
• OpenTelemetry Metrics Opt-Out: A new mechanism has been introduced to disable the automatic injection of OpenTelemetry meters for grpc-gcp metrics. This can be controlled via SpannerOptions or the SPANNER_DISABLE_GRPC_GCP_OTEL_METRICS environment variable.
• Unit Tests: Comprehensive unit tests have been added to validate both the channel configurator preservation and the new OpenTelemetry metrics opt-out functionality.

Changelog

• google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java
  • Added a new enableGrpcGcpOtelMetrics field to SpannerOptions and its Builder to control OpenTelemetry metrics injection.
  • Introduced isEnableGrpcGcpOtelMetrics() methods in SpannerEnvironment and SpannerEnvironmentImpl to read the new setting, defaulting to true and allowing override via SPANNER_DISABLE_GRPC_GCP_OTEL_METRICS environment variable.
  • Added a public setGrpcGcpOtelMetricsEnabled() method to the Builder for programmatic control.
  • Added a getter isGrpcGcpOtelMetricsEnabled() to SpannerOptions.
• google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpc.java
  • Modified the grpcGcpOptionsWithMetricsAndDcp method to only inject an OpenTelemetry meter if options.isGrpcGcpOtelMetricsEnabled() is true and no meter is already set.
  • Updated maybeEnableGrpcGcpExtension to explicitly retrieve and apply the base ChannelConfigurator from defaultChannelProviderBuilder before applying grpc-gcp specific configurations, ensuring its preservation.
• google-cloud-spanner/src/test/java/com/google/cloud/spanner/spi/v1/GapicSpannerRpcTest.java
  • Added testGrpcGcpExtensionPreservesChannelConfigurator to verify that the original channel configurator is called when grpc-gcp is enabled.
  • Added testGrpcGcpOtelMetricsDisabledSkipsMeterInjection to confirm that OpenTelemetry meters are not injected when the new opt-out setting is active.
  • Included necessary imports for new test classes and methods.

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[gemini-code-assist]

Code Review

This pull request aims to add an opt-out mechanism for grpc-gcp OpenTelemetry metrics. However, it introduces a critical Denial of Service (DoS) vulnerability due to an infinite recursion bug in the channel configurator logic within GapicSpannerRpc.java. The current implementation incorrectly attempts to retrieve the configurator from the builder inside the lambda, leading to a StackOverflowError and application crash. This can be remediated by capturing the existing configurator in a local variable before defining the new lambda.