Overhaul stats: Add new metric to the `metrics` endpoint with the current number of banned IPs

[josecelano]

Relates to: torrust/torrust-demo#50

On the stats endpoint, there is one field , udp_banned_ips_total, that I forgot to include in the new metrics endpoint.

We need to add it to the metrics endpoint

{
  "torrents": 308709,
  "seeders": 160858,
  "completed": 1017531,
  "leechers": 233351,
  "tcp4_connections_handled": 0,
  "tcp4_announces_handled": 0,
  "tcp4_scrapes_handled": 0,
  "tcp6_connections_handled": 0,
  "tcp6_announces_handled": 0,
  "tcp6_scrapes_handled": 0,
  "udp_requests_aborted": 120628,
  "udp_requests_banned": 21852552,
  "udp_banned_ips_total": 126064, <- this one
  "udp_avg_connect_processing_time_ns": 925261,
  "udp_avg_announce_processing_time_ns": 1687830,
  "udp_avg_scrape_processing_time_ns": 207516,
  "udp4_requests": 333519868,
  "udp4_connections_handled": 184164697,
  "udp4_announces_handled": 119644012,
  "udp4_scrapes_handled": 771000,
  "udp4_responses": 305253228,
  "udp4_errors_handled": 971898,
  "udp6_requests": 0,
  "udp6_connections_handled": 0,
  "udp6_announces_handled": 0,
  "udp6_scrapes_handled": 0,
  "udp6_responses": 0,
  "udp6_errors_handled": 0
}

• stats endpoint: http://localhost:1212/api/v1/stats?token=MyAccessToken&format=prometheus`
• metrics endpoint: http://localhost:1212/api/v1/metrics?token=MyAccessToken&format=prometheus`