impl(bigtable): add DynamicChannelPool class

[scotthart]

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[gemini-code-assist]

Summary of Changes

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This pull request introduces a significant enhancement to how Bigtable clients manage their gRPC connections. By implementing a DynamicChannelPool, the system can now intelligently scale the number of active gRPC channels up or down based on the current RPC load. This dynamic adjustment aims to optimize resource utilization, reduce latency during peak loads, and improve overall efficiency for Bigtable operations by ensuring an appropriate number of channels are available without over-provisioning.

Highlights

• Dynamic Channel Pool Introduction: Introduced a new DynamicChannelPool class designed to manage gRPC channels for Bigtable, enabling dynamic scaling based on current RPC load.
• Configurable Sizing Policy: Implemented a DynamicChannelPoolSizingPolicy structure, allowing configuration of how the channel pool grows and shrinks, including cooldown intervals and thresholds for average outstanding RPCs.
• Unit Tests for Dynamic Behavior: Added comprehensive unit tests for the DynamicChannelPool to validate its functionality, channel selection logic, and dynamic resizing behavior under various conditions.
• Build System Integration: Updated CMake and Bazel build configurations to properly include the new dynamic_channel_pool.h header and dynamic_channel_pool_test.cc source file.
• ChannelUsage Enhancements: Modified channel_usage.h to include a testing-specific constructor and adjusted the average_outstanding_rpcs logic to support initial RPC counts for test scenarios.

Changelog

• google/cloud/bigtable/CMakeLists.txt
  • Added internal/dynamic_channel_pool.h to the library sources.
  • Added internal/dynamic_channel_pool_test.cc to the test sources.
• google/cloud/bigtable/bigtable_client_unit_tests.bzl
  • Added internal/dynamic_channel_pool_test.cc to the list of unit tests.
• google/cloud/bigtable/google_cloud_cpp_bigtable.bzl
  • Added internal/dynamic_channel_pool.h to the list of header files.
• google/cloud/bigtable/internal/channel_usage.h
  • Added a constructor for testing purposes that allows setting an initial outstanding RPC count.
  • Adjusted average_outstanding_rpcs to account for initial RPC counts in testing.
  • Added std::cout statements for debugging in AcquireStub and the new testing constructor.
• google/cloud/bigtable/internal/dynamic_channel_pool.h
  • Implemented the DynamicChannelPool class, which manages a pool of ChannelUsage objects and dynamically adjusts its size based on RPC load.
  • Defined DynamicChannelPoolSizingPolicy to configure pool behavior.
• google/cloud/bigtable/internal/dynamic_channel_pool_test.cc
  • Added comprehensive unit tests for the DynamicChannelPool class, covering channel selection, pool resizing, and handling of empty/single-channel pools.

Activity

• No human activity has been recorded on this pull request yet.

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[codecov]

Codecov Report

❌ Patch coverage is 70.31963% with 130 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.60%. Comparing base (8989a78) to head (1a1c5ee).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...gle/cloud/bigtable/internal/dynamic_channel_pool.h	48.49%	120 Missing ⚠️
...oud/bigtable/internal/dynamic_channel_pool_test.cc	94.87%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16037      +/-   ##
==========================================
- Coverage   92.64%   92.60%   -0.04%     
==========================================
  Files        2337     2339       +2     
  Lines      214938   215375     +437     
==========================================
+ Hits       199125   199457     +332     
- Misses      15813    15918     +105     

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