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[bot]]

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