Add basic prometheus metrics

[Thor77]

This is a quick draft for adding some basic Prometheus metrics at /metrics if the -metrics flag is passed.

If you're open to adding metrics in general I would add some more metrics for the remaining error conditions and hook execution time.

In case you don't like the addition of all those dependencies I could also switch the implementation to use https://github.com/VictoriaMetrics/metrics which doesn't have as many useful features but is also a lot smaller.

Current custom metrics example:

# HELP webhook_hook_errors_total Total number of hook errors
# TYPE webhook_hook_errors_total counter
webhook_hook_errors_total{error="rules",hook="deploy"} 1
# HELP webhook_hook_requests_total Total number of hook requests
# TYPE webhook_hook_requests_total counter
webhook_hook_requests_total{hook="deploy"} 3
# HELP webhook_hook_success_total Total number of successful hook executions
# TYPE webhook_hook_success_total counter
webhook_hook_success_total{hook="deploy"} 2

Solves #527