Bigtable dynamic channel pool prototype

[scotthart]

[gemini-code-assist[bot]]

Summary of Changes

Hello @scotthart, 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 prototypes a dynamic channel pooling system for Bigtable clients, designed to enhance load balancing and resource management. It introduces a "random two least used" strategy for selecting gRPC channels, aiming to distribute RPCs more efficiently across available connections. The changes involve new internal components for channel management, modifications to the stub factory to integrate this new strategy, and extensive debugging output to monitor its behavior.

Highlights

• Dynamic Channel Pool Introduction: A new DynamicChannelPool mechanism is introduced to manage Bigtable stubs, allowing for dynamic scaling of channels based on load.
• Random Two Least Used Strategy: A BigtableRandomTwoLeastUsed decorator is implemented, which selects a channel from the pool using the "random two least used" load balancing strategy.
• Integration with Stub Factory: The Bigtable stub factory is updated to utilize this new BigtableRandomTwoLeastUsed decorator, replacing the previous round-robin approach for channel selection.
• Channel Activity Tracking: StubWrapper and StreamingReadRpcTracking/AsyncStreamingReadRpcTracking classes are added to track the number of outstanding RPCs on each channel, which is crucial for the "least used" strategy.
• Debugging Output: Numerous std::cout statements have been added across various Bigtable internal files to trace the execution flow, likely for debugging and understanding the new channel pooling behavior.

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 67.82178% with 130 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 92.91%. Comparing base (826dd38) to head (59d22fb). :warning: Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...gle/cloud/bigtable/internal/dynamic_channel_pool.h	47.10%	128 Missing :warning:
...e/cloud/bigtable/internal/bigtable_stub_factory.cc	95.91%	2 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15819      +/-   ##
==========================================
- Coverage   92.95%   92.91%   -0.05%     
==========================================
  Files        2458     2460       +2     
  Lines      227589   227977     +388     
==========================================
+ Hits       211547   211814     +267     
- Misses      16042    16163     +121     

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