feat(server): add /metrics endpoint

[crrow]

Problem

docs/design/layout.md promises a /metrics endpoint but no metrics route exists in any service.

Acceptance Criteria

• Add Prometheus-compatible /metrics endpoint to the server crate
• Integrate metrics + metrics-exporter-prometheus crates
• Register basic HTTP request metrics (count, latency histogram)
• Endpoint available on all services

References

• docs/design/layout.md:60