impl(bigtable): update stub_factory and connection creation functions for dynamic pool

[scotthart]

Adds new functions and branch to create stubs using the DynamicChannelPool.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the Bigtable client library by introducing support for dynamic channel pooling and instance affinity. This allows the client to maintain separate, optimized connections for different Bigtable instances, potentially improving performance and resource utilization by priming connections and using a more intelligent channel selection strategy (random two least used) when instance affinity is enabled.

Highlights

• Dynamic Channel Pooling: Introduced an experimental::InstanceChannelAffinityOption that, when set, enables the creation of a dynamic channel pool for Bigtable connections, allowing for instance-specific channel affinity and priming.
• Conditional Stub Creation: Modified MakeDataConnection to conditionally create DataConnectionImpl using a StubManager that leverages the new dynamic channel pool if the InstanceChannelAffinityOption is configured, otherwise falling back to the existing round-robin stub creation.
• New Stub Factory Functions: Added new stub factory functions, including CreateBigtableStubRandomTwoLeastUsed and overloaded CreateBigtableStub and CreateBigtableAffinityStubs, to support the creation and management of stubs within the dynamic channel pool, including PingAndWarm priming.
• Code Refactoring: Refactored common stub decorators into a new helper function ApplyCommonDecorators to reduce code duplication and improve maintainability across different stub creation paths.
• New Test Coverage: Added new test cases to data_connection_test.cc and bigtable_stub_factory_test.cc to verify the functionality of the InstanceChannelAffinityOption, the RandomTwoLeastUsed channel selection strategy, and the CreateBigtableAffinityStubs function.

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[gemini-code-assist]

Code Review

This pull request introduces support for a dynamic channel pool with instance affinity, a significant enhancement for managing Bigtable connections. The changes are well-structured, introducing a new experimental option InstanceChannelAffinityOption to control the feature. The implementation correctly branches based on this option in MakeDataConnection, preserving the existing behavior for users who do not opt-in. The introduction of StubManager provides a clean abstraction for managing stubs, whether it's a single shared stub or multiple affinity-based stubs. The new BigtableRandomTwoLeastUsed decorator and DynamicChannelPool implement the core logic for the new connection management strategy. The code is well-refactored, for example, by extracting common decorator logic into ApplyCommonDecorators, which improves maintainability. The changes are also accompanied by thorough tests, covering both the new logic and ensuring existing functionality is not broken. Overall, this is a high-quality contribution that thoughtfully adds a powerful new feature.

[codecov]

Codecov Report

❌ Patch coverage is 96.70330% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.69%. Comparing base (eb60077) to head (c5cefff).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/bigtable_stub_factory.cc	93.10%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16052    +/-   ##
========================================
  Coverage   92.68%   92.69%            
========================================
  Files        2342     2342            
  Lines      216379   216537   +158     
========================================
+ Hits       200554   200709   +155     
- Misses      15825    15828     +3     

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[diegomarquezp]

LGTM overall, just a minor comment