impl(bigtable): add RandomTwoLeastUsed stub decorator

[scotthart]

Adds a BigtableStub decorator that holds the DynamicChannelPool object and wraps calls to the child stub with logic to increment and decrement the ChannelUsage RPC counter.

[gemini-code-assist]

Summary of Changes

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This pull request introduces a new BigtableRandomTwoLeastUsed stub decorator for the Bigtable client library. This decorator is designed to work with the DynamicChannelPool to implement a load-balancing strategy that tracks the usage of underlying gRPC channels. By wrapping RPC calls, it ensures that channel usage is correctly accounted for, allowing the system to select the two least used channels for subsequent requests, thereby optimizing resource distribution.

Highlights

• New Stub Decorator: Added a BigtableRandomTwoLeastUsed stub decorator to manage Bigtable RPCs.
• Dynamic Channel Pool Integration: The decorator integrates with DynamicChannelPool to select channels based on usage.
• Channel Usage Tracking: Implemented logic to increment and decrement ChannelUsage RPC counters for effective load balancing.

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[gemini-code-assist]

Code Review

This pull request introduces a BigtableRandomTwoLeastUsed stub decorator. This decorator uses a DynamicChannelPool to select a stub for each RPC using a 'random two least used' strategy, and it correctly manages the usage count of stubs via AcquireStub and ReleaseStub calls. The implementation is well-structured, using helper templates to reduce boilerplate for unary, streaming, and async streaming RPCs. The new functionality is also accompanied by unit tests.

My main feedback is a suggestion to improve the robustness of the UnaryHelper function by using an RAII pattern to ensure resources are released even in the case of an exception, aligning it with the safer implementation of the streaming helpers.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.67%. Comparing base (98cabd9) to head (e821573).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16049      +/-   ##
==========================================
- Coverage   92.68%   92.67%   -0.01%     
==========================================
  Files        2341     2342       +1     
  Lines      216206   216344     +138     
==========================================
+ Hits       200391   200503     +112     
- Misses      15815    15841      +26     

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