expose glider metrics for monitoring purposes

[spinus]

Would be very nice to be able to monitor what glider is doing internally to say whether is working as expected.

Either exporting prometheus metrics (on http port) or printing them on linux signal (ex, USR1) would be nice feature.

Proposed set of metrics:

• for each defined proxy
  glider_proxy_defined{type="forward|listen|...", protocol="socks5|..."}
  glider_proxy_healthcheck_works{type="forward|...", website="google.com", protocol="socks4|..."} 1
  glider_proxy_query_time_sum{...} # prometheus histogram type
  glider_proxy_query_time{le="5.0"}
  glider_proxy_errors{...error_code="404"} # number of errors

[github-actions]

This issue is stale because it has been open 90 days with no activity. Remove stale label or comment or this will be closed in 5 days.