Add observability to RouteCacheKey::for_route_request

[coderabbitai]

Background

Introduced in PR #346, RouteCacheKey::for_route_request derives canonical cache keys from route request payloads. The implementation has no logging, tracing spans, or metrics.

Problem

Operators cannot:

• Diagnose derivation failures in production
• Measure derivation latency or detect regressions
• Monitor error rates for RouteCacheKeyDerivationError

Required work

1. Add a tracing::debug_span (or #[tracing::instrument]) at the for_route_request entry point recording input size and outcome.
2. Emit a tracing::warn! log when RouteCacheKeyDerivationError is returned, including a stable contextual message.
3. Enrich RouteCacheKeyDerivationError variants with contextual display messages suitable for log output.
4. Add a metrics counter (using the existing Prometheus infrastructure) for derivation errors and a histogram for derivation latency.

References

• PR Add canonical route cache key derivation and tests (5.1.4) #346 (Add canonical route cache key derivation and tests (5.1.4) #346)
• Requested by @leynos in PR Add canonical route cache key derivation and tests (5.1.4) #346 review