feat(spanner): add dynamic channel pool support

[rahul2393]

Add Go Spanner Dynamic Channel Pooling (DCP) with scale-up/down, drain safety, DirectPath fallback support,
observability.

• Adds opt-in DynamicChannelPoolConfig for Go Spanner.
• Implements DCP picker, scale-up, scale-down, draining, priming, and error penalty.
• Preserves transaction/session stickiness via operationRefs.
• Supports DirectPath fallback with paired wrapper slots.
• Adds OTel-only DCP metrics for debugging.

Internal reference: go/go-dcp-design

[gemini-code-assist]

Code Review

This pull request implements a Dynamic Channel Pool (DCP) for the Spanner client, which automatically scales the number of gRPC channels based on RPC load. The implementation includes features like Power-of-Two-Least-Busy selection, channel priming, graceful draining, and OpenTelemetry metrics. Review feedback highlights the need to improve configuration defaulting for initial channels and to propagate request contexts through the session management layer to ensure consistent observability and trace propagation.

[gemini-code-assist]

The current defaulting logic for DCPInitialChannels can lead to validation failures if a user provides a DCPMinChannels value greater than the default initial channels (4). Consider defaulting DCPInitialChannels to at least DCPMinChannels to ensure consistency, while ensuring that this change does not break backward compatibility for existing stable code paths.

Suggested change

	DCPMaxRemoveChannels: 2,
	DCPDrainIdleGrace: 5 * time.Minute,
	DCPMaxDrainTimeout: 30 * time.Minute,
	if cfg.DCPInitialChannels == 0 {
	cfg.DCPInitialChannels = def.DCPInitialChannels
	if cfg.DCPInitialChannels < cfg.DCPMinChannels {
	cfg.DCPInitialChannels = cfg.DCPMinChannels
	}
	}

References

1. When introducing new components with different default behaviors, avoid altering the defaults of existing, stable code paths to prevent breaking changes for backward compatibility.