feat(spanner): add experimental numChannels round-robin transport mode

[rahul2393]

Summary

Adds an experimental fixed-channel round-robin mode for Spanner data RPCs.

By default, when no customer grpc.gcpApiConfig is provided, Spanner now uses numChannels = 4 and disables grpc-gcp channel pooling for Spanner data RPCs. Customers can override the channel count with numChannels, or continue using grpc-gcp by passing a custom grpc.gcpApiConfig.

When fixed-channel mode is active:

• grpc-gcp call transformer/channel factory config is disabled
• grpc.use_local_subchannel_pool = 1 is set so grpc-js does not share one transport
• Spanner data clients are keyed by channel index
• non-transactional Spanner RPCs use round-robin channel selection
• transaction RPCs keep a per-transaction channel hint so begin/read/query/commit/rollback stay on the same channel

Motivation

Multiplexed sessions currently bind transaction RPCs to the session affinity key under grpc-gcp, which can pin high-concurrency write workloads to one HTTP/2 transport. Fixed-channel mode enables round-robin distribution while preserving transaction stickiness.

Validation

Added local benchmark tooling with Go mock Spanner server:

• 110 concurrent table.insert() calls
• 50ms injected Commit latency
• mux session default

Observed warmed run with default numChannels = 4:

• P50: ~94.65ms
• P90: ~94.99ms
• P99: ~95.17ms
• 4 server-side transports
• Commit distribution: 27/27/28/28
• 110/110 transaction affinity hits
• 0 affinity misses

Observed warmed run with SPANNER_NUM_CHANNELS=10:

• P50: ~90.60ms
• P90: ~91.08ms
• P99: ~91.61ms
• 10 server-side transports
• 11 Commit RPCs per transport
• 110/110 transaction affinity hits
• 0 affinity misses

Notes

This is experimental and currently focused on Spanner data RPC routing. Admin clients continue to use normal shared client behavior. Customers with custom grpc-gcp configs can keep the grpc-gcp path by passing grpc.gcpApiConfig.

[gemini-code-assist]

Code Review

This pull request introduces an experimental numChannels option to the Spanner client, enabling manual gRPC channel pooling and round-robin assignment for transaction RPCs as an alternative to grpc-gcp. The changes involve managing multiple internal client instances within the Spanner class and ensuring that transactions consistently use the same channel via a channelHint. Feedback was provided regarding the round-robin counter implementation to avoid potential precision loss by applying the modulo operation during increment.