Add Prometheus /metrics HTTP endpoint

Wire up unused metrics_port/metrics_whitelist config into working
HTTP server exposing proxy stats in Prometheus text format.

docker-compose.yml

@@ -5,6 +5,7 @@ services:
     restart: unless-stopped
     ports:
       - "443:443"
+      - "9090:9090"
     # Allow caching 'proxy-secret' in read-only container
     working_dir: /run/telemt
     volumes: