feat: make grpc-gcp default enabled

[rahul2393]

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Fixes #<issue_number_goes_here> ☕️

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[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 transitions the grpc-gcp extension in the Spanner client library to be enabled by default. This change simplifies the client's internal logic by removing a configuration flag and its conditional code paths, ensuring that all users benefit from the performance and reliability improvements offered by gRPC-GCP without requiring manual activation.

Highlights

• Default gRPC-GCP Enablement: The grpc-gcp extension is now enabled by default, removing the need for explicit configuration to activate it.
• Code Simplification: The grpcGcpExtensionEnabled boolean field and all associated conditional logic have been removed from SpannerOptions and GapicSpannerRpc, streamlining the codebase.
• Consistent Channel Management: Channel affinity and the number of channels are now consistently managed under the assumption that gRPC-GCP is always active, simplifying channel configuration.

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[gemini-code-assist]

Code Review

This pull request makes the grpc-gcp extension enabled by default, which is a good simplification. The changes correctly remove the feature flag and update the related logic. My review includes a few suggestions to improve API clarity and test naming following these changes:

• API Clarity: The methods enableGrpcGcpExtension and disableGrpcGcpExtension are now misleading or no-ops. I've suggested addressing this to avoid confusion.
• Test Naming: Some test method names have become outdated due to the changes. I've pointed these out with suggestions for new names that better reflect their current purpose.

[gemini-code-assist]

This line is correctly removed. However, with gRPC-GCP now enabled by default, the method name enableGrpcGcpExtension is misleading as it only configures options. Consider deprecating this method and introducing a new one with a more descriptive name like setGrpcGcpOptions to improve API clarity.

[gemini-code-assist]

While removing this line is correct as part of making gRPC-GCP always enabled, the disableGrpcGcpExtension() method now has no effect. This can be misleading for users. The method should be deprecated to clearly communicate that it's a no-op and the extension cannot be disabled.

It would be best to modify the method on the right side to be:

    /**
     * Disables gRPC-GCP extension.
     *
     * @deprecated gRPC-GCP is now enabled by default and can no longer be disabled. This method is a no-op.
     */
    @java.lang.Deprecated
    public Builder disableGrpcGcpExtension() {
      return this;
    }

[gemini-code-assist]

The name of this test, testDefaultNumChannelsWithGrpcGcpExtensionDisabled, is now misleading because the gRPC-GCP extension can no longer be disabled. Since this test now verifies the behavior when the no-op disableGrpcGcpExtension() method is called, consider renaming it to something like testDefaultNumChannelsWhenDisableGrpcGcpExtensionIsCalled for clarity.

[gemini-code-assist]

The name of this test, checkCreatedInstanceWhenGrpcGcpExtensionDisabled, is now misleading as the gRPC-GCP extension is always enabled. To improve clarity, consider renaming the test to reflect that it's verifying behavior when the no-op disableGrpcGcpExtension() is called, for example: checkCreatedInstanceWhenDisableGrpcGcpExtensionIsCalled.

[sakthivelmanii]

@rahul2393 as a first step, can you please enable it by default without removing the option? If needed, customer can disable it.