chore(spanner): implement latency-aware replica selection and scoped tracking

[olavloite]

Implement Power of Two Choices (P2C) replica selection in LocationRouter weighted by EWMA latency and active in-flight request count:

• Encapsulate database_scope: String within LocationRouter so recording and replica cost queries are strictly and immutably scoped to the database.
• Add connection presence and health checks to is_tablet_routable so P2C only considers pre-warmed and ready candidate replicas, avoiding false fallbacks to the default gateway.
• Expose record_latency and record_routing_error on DatabaseClient.

[gemini-code-assist]

Code Review

This pull request integrates the LatencyRegistry and a Power of Two Choices (P2C) replica selector into the LocationRouter and DatabaseClient to enable latency-aware routing for Spanner. It introduces methods to record latency and routing errors, and updates connection resolution to select healthy replicas weighted by EWMA latency and active request counts. Feedback suggests optimizing select_healthy_tablet to perform a single lookup in the ConnectionCache instead of querying it twice per replica, which reduces lock contention.

[codecov]

Codecov Report

❌ Patch coverage is 99.44979% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.60%. Comparing base (2808196) to head (815ac0e).

Files with missing lines	Patch %	Lines
src/spanner/src/routing/key_range_cache.rs	0.00%	3 Missing ⚠️
src/spanner/src/routing/location_router.rs	99.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6572      +/-   ##
==========================================
+ Coverage   96.58%   96.60%   +0.01%     
==========================================
  Files         304      304              
  Lines       91285    91987     +702     
==========================================
+ Hits        88165    88861     +696     
- Misses       3120     3126       +6     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates latency-aware routing and replica selection into the Spanner database client. It introduces a LatencyRegistry and uses a Power of Two Choices (P2C) selector within the LocationRouter to dynamically route requests to healthy replicas based on EWMA latency and active in-flight requests. Feedback is provided to optimize the hot path in select_healthy_replica by sampling candidate indices first, which avoids heap allocations and unnecessary connection cloning.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates latency-aware routing into the Spanner database client. It introduces latency and error penalty recording on the DatabaseClient and updates the LocationRouter to perform Power-of-Two-Choice (P2C) replica selection weighted by EWMA latency and active in-flight requests. Key updates include the addition of LatencyRegistry and PowerOfTwoSelector integrations, stack-allocated candidate buffers to avoid heap allocations on the hot path, and comprehensive unit tests. Feedback on the changes suggests defining a constant for the magic number 8 used in the stack buffer array initialization and using a slice instead of a fixed-size array reference in select_p2c_winner to make the code more idiomatic.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates latency-aware routing and replica selection into the Spanner client. It introduces a LatencyRegistry to track round-trip latency samples and RPC error penalties scoped by database. The LocationRouter is updated to perform Power-of-Two-Choice (P2C) replica selection among healthy follower replicas, weighted by EWMA latency and active in-flight requests, using a stack-allocated buffer to avoid heap allocations on the hot path. Comprehensive unit and integration tests are added to verify the routing, latency recording, and P2C selection behavior. I have no feedback to provide as the implementation is clean, well-tested, and conforms to the repository's style guidelines.