feat: Add Dynamic Channel Pooling (DCP) support to Connection API

[rahul2393]

Summary

• Add connection URL properties to enable and configure Dynamic Channel Pooling (DCP)
• Allow JDBC and Connection API users to opt-in to automatic gRPC channel scaling
• Integrate DCP settings into SpannerPoolKey for proper connection pooling behavior

Details

This PR exposes the Dynamic Channel Pooling feature (already available in SpannerOptions)
through the Connection API and JDBC driver via connection URL properties.

New Connection Properties

Property	Type	Default	Description
enableDynamicChannelPool	boolean	null (uses SpannerOptions default: false)	Enable dynamic channel pooling
dcpMinChannels	int	null (uses SpannerOptions default: 2)	Minimum channels in pool
dcpMaxChannels	int	null (uses SpannerOptions default: 10)	Maximum channels in pool
dcpInitialChannels	int	null (uses SpannerOptions default: 4)	Initial channels at startup

Example Usage

jdbc:cloudspanner:/projects/my-project/instances/my-instance/databases/my-db?enableDynamicChannelPool=true;dcpMinChannels=3;dcpMaxChannels=15

Important Notes

• Setting numChannels disables DCP even if enableDynamicChannelPool=true
• DCP requires gRPC-GCP extension (enabled by default)
• When custom DCP channel options are specified, other DCP settings (RPC thresholds, intervals) use SpannerOptions defaults

[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 introduces support for Dynamic Channel Pooling (DCP) within the Connection API, providing a mechanism for the Spanner client to automatically adjust the number of gRPC channels based on real-time load. This feature aims to optimize resource utilization and improve performance by ensuring that the client can scale its channel capacity up or down as needed, without manual intervention.

Highlights

• Dynamic Channel Pooling (DCP) Configuration: Introduced new connection properties (enableDynamicChannelPool, dcpMinChannels, dcpMaxChannels, dcpInitialChannels) to allow users to configure dynamic gRPC channel scaling directly through the Connection API.
• Spanner Client Integration: Integrated these new DCP properties into the SpannerPool to dynamically configure the underlying SpannerOptions for gRPC channel management, ensuring that channel scaling adapts to application load.
• Comprehensive Testing: Added extensive unit tests for ConnectionOptions to validate the parsing of new DCP properties from connection URIs and for SpannerPool to confirm that different DCP settings correctly influence the pooling key and Spanner client creation.

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[gemini-code-assist]

Code Review

This pull request adds support for Dynamic Channel Pooling (DCP) to the Spanner Connection API. The changes are well-implemented and include new connection properties, logic to configure the Spanner client with DCP settings, and corresponding tests. I have one suggestion to improve the maintainability of the code that configures the DCP options.

[olavloite]

nit: Would you mind adding something like `The default is SpannerOptions.DEFAULT_ENABLE_DYNAMIC_CHANNEL_POOL (false). This default will be changed to true in a future version.'? (At least, assuming that this is correct)

[olavloite]

This assumes that dynamic channel pooling is disabled by default. This is currently correct, but is likely to change in the future. Can we therefore either:

1. Add explicit behavior here to disable DCP if enableDynamicChannelPool has been set to false?
2. Or add a test to the Connection API that will fail if the default setting for DCP in the client library changes?

What I mean is the following:

1. A user that sets enableDynamicChannelPool=true should always get DCP. This works fine now.
2. A user that does not set a value for enableDynamicChannelPool should get the default of the client library. That works fine now, and the default is that it is disabled.
3. A user that sets enableDynamicChannelPool=false should always get a fixed channel pool. That works now, but if the default in the client library changes, then they would all of a sudden get DCP.

[olavloite]

There should also be a test for testDisableDynamicChannelPool