feat: add ChannelFinder server interfaces

[rahul2393]

This commit adds the server abstraction interfaces for location-aware routing:

• ChannelFinderServer: Interface representing a Spanner server endpoint with address, health check, and channel access
• ChannelFinderServerFactory: Factory interface for creating and caching server connections
• GrpcChannelFinderServerFactory: gRPC implementation that creates and manages gRPC channels for different server endpoints

These interfaces enable the client to maintain connections to multiple Spanner servers and route requests directly to the appropriate server based on key location information.

This is part of the experimental location-aware routing for improved latency.

[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 foundational interfaces and a gRPC-specific implementation to support an experimental location-aware routing feature within the Spanner client library. The new components enable the client to establish and manage connections to multiple Spanner server endpoints, facilitating intelligent request routing based on key location information. This enhancement is designed to improve overall latency by directing requests to the most geographically appropriate server.

Highlights

• New Interface: ChannelFinderServer: Introduced the ChannelFinderServer interface, which defines a Spanner server endpoint. It includes methods to retrieve the server's address, check its health status, and access its underlying gRPC ManagedChannel.
• New Interface: ChannelFinderServerFactory: Added the ChannelFinderServerFactory interface, designed to abstract the creation and caching mechanisms for ChannelFinderServer instances, providing a way to manage server connections.
• New Implementation: GrpcChannelFinderServerFactory: Implemented GrpcChannelFinderServerFactory, a concrete gRPC-based factory that adheres to the ChannelFinderServerFactory interface. This class manages ManagedChannel connections to various Spanner server endpoints, supporting both a default server and dynamic creation for specific addresses, utilizing InstantiatingGrpcChannelProvider.Builder.

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[gemini-code-assist]

Code Review

This pull request introduces new interfaces and an implementation for location-aware routing, which is a great step towards improving latency. The new abstractions ChannelFinderServer and ChannelFinderServerFactory are well-defined. My review includes suggestions for improving the Javadoc for these new interfaces to make them clearer. I've also identified a potential concurrency issue in GrpcChannelFinderServerFactory where a shared builder is mutated, and I've proposed a fix. Additionally, there are some minor points on error handling and code clarity.

[gemini-code-assist]

The methods in this interface should have Javadoc comments to explain their purpose, expected inputs, and what they return. This will improve the maintainability and usability of this new API.

  /** Returns the default server that is used if no specific server can be found for a request. */
  ChannelFinderServer defaultServer();

  /**
   * Creates or returns a cached server for the given address.
   *
   * @param address The address of the server to connect to.
   * @return A {@link ChannelFinderServer} for the given address.
   */
  ChannelFinderServer create(String address);