chore(spanner): wire channel pool into client

[olavloite]

• Wire ChannelPool into the Spanner client and builder via SpannerPoolBuilderExt.
• Support configuring static and dynamic channel pools through extensions, SPANNER_NUM_CHANNELS, or emulator defaults.
• Dial initial pool channels concurrently during client construction.
• Add pick_channel and resolve_affinity on Spanner for channel selection.
• Update SessionMaintainer and CacheSubscriber to lease channels from the pool.

[gemini-code-assist]

Code Review

This pull request integrates the ChannelPool into the Spanner client, replacing manual channel management with a robust pool that supports both static and dynamic configurations. It updates various client methods and routing logic to lease channels from the pool using pick_channel and resolve_affinity, and adds comprehensive tests for pool builder extensions and configuration resolution. A critical issue was identified in ActiveRpcGuard's implementation of StreamGuard::record_error_code, which currently causes infinite recursion and must be updated to delegate to the underlying entry.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates the ChannelPool into the Spanner client, replacing the previous round-robin channel selection with a robust channel pool mechanism. It introduces support for both static and dynamic channel pool configurations, enabling concurrent channel dialing during client initialization via JoinSet. Additionally, it updates client operations, routing, and session maintenance to utilize channel leases and record RPC call results for health and penalty tracking. There are no review comments provided, so I have no feedback to offer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.06%. Comparing base (597aa88) to head (8b487c3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6793      +/-   ##
==========================================
+ Coverage   97.01%   97.06%   +0.04%     
==========================================
  Files         326      326              
  Lines      107548   107976     +428     
==========================================
+ Hits       104342   104808     +466     
+ Misses       3206     3168      -38     

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