feat(bigtable): introduce prime() method and load counting based on unary,streaming and ability to remove connection

[sushanb]

no-op from https://github.com/sushanb/google-cloud-go/pull/1

[gemini-code-assist[bot]]

Summary of Changes

Hello @sushanb, I'm Gemini Code Assist[^1]! 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 significantly refactors the Bigtable client's internal connection pooling mechanism. The primary goal is to enhance connection management by introducing a Prime method to warm up connections, refining load balancing through distinct tracking of unary and streaming requests, and implementing a robust graceful draining process for connections. These changes aim to improve the client's performance, reliability, and responsiveness by ensuring connections are efficiently utilized and gracefully managed throughout their lifecycle.

Highlights

• Connection Priming: Introduced a Prime() method for Bigtable connections to proactively warm them up using a PingAndWarm RPC, reducing initial request latency.
• Granular Load Tracking: Implemented separate atomic counters for in-flight unary and streaming requests (unaryLoad, streamingLoad) within each connection entry, allowing for more precise load balancing decisions.
• Graceful Connection Draining: Added functionality to gracefully drain connections when they are being removed or replaced. Connections are marked as draining and new requests are routed to other connections, while existing in-flight requests are allowed to complete within a configurable timeout.
• Enhanced Connection Pool Options: The NewBigtableChannelPool constructor now accepts a context.Context and new options to configure the pool with an app profile, instance name, feature flags metadata, logger, and meter provider.
• Improved Selection Strategies: Load balancing selection methods (Round Robin, Least In-Flight, Power of Two Least In-Flight) have been updated to skip connections that are in a draining state, ensuring requests are always sent to active connections.

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