Add metrics route for Prometheus

Add /metrics route for Prometheus, consider RED, USE and LETS metrics:

• Check Prometheus client;
• ✅ Basic CMS metrics: #204 (comment 2445768183);
• ✅ Basic runtime metrics via Telemetry module: #204 (comment 2433801386);
• ✅ $*KERNEL class: #204 (comment 2436251584);
• ✅ FastCGI environment: #204 (comment 2436626082);
• ✅ Data from Pheix::Controller::Stats per week: hosts and visits: #204 (comment 2437486383);
• ✅ Data from log: entries, logons, etc...: #204 (comment 2441105065), #204 (comment 2441895053), #204 (comment 2443195034).

Dashboard: https://pheix.org.ru/grafana/public-dashboards/0ce87e9f62c94560b082bf8a38ddb588