Add metrics and observability endpoints (Prometheus/OpenTelemetry)

[szaffarano]

Feature Request: Observability and Metrics

Add endpoints suitable for Prometheus scraping / OpenTelemetry export:

• HTTP metrics endpoint (/metrics), with at least counters for requests, errors, latency, auth failures, etc.
• Document recommended setup for enabling metrics.
• Consider config to enable/disable metrics for privacy reasons.

Improved observability enables production readiness and easier troubleshooting.