Add service-route observability

[scotwells]

Summary

Add bounded-cardinality observability for the node-local service-route controller and dataplane programming.

What this adds

• Prometheus metrics for reconciliation outcomes and duration.
• Prometheus metrics for route apply/remove operations.
• A gauge for locally programmed route intents.
• Structured logs with policy, endpoint, node, intent count, and failure context.
• Metrics registration on the existing Galactic router metrics endpoint.

Resource names are intentionally kept out of metric labels to avoid unbounded cardinality at large tenant counts. Detailed identities remain in structured logs.

Validation

• go test ./internal/serviceroute

The broader controller package requires Linux-only netlink/eBPF APIs and remains covered by Linux CI.

This PR targets the service-route implementation branch and should merge after the dataplane PR.