Adds a dedicated Prometheus metrics endpoint on port 9098 (configurable
via METRICS_PORT env var) with counters for HTTP requests, histograms for
request duration, gauges for uptime and active WebSocket connections, and
counters for WebSocket messages. Metrics are registered using once_cell
Lazy statics with abort-on-failure to satisfy the crate's deny(unwrap)
lint policy.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>