add prometheus scraping for docs.rs nginx metrics

[syphar]

There were some issues about open-fd count that lead to a bunch of server errors.

this is based on the integrated nginx stub_status and nginx-prometheus-exporter which is running via systemd.

This will give us metrics around connection & FD-count, among others:
https://docs.rs/about/metrics/gwlfgmnhpddteabp/nginx

[syphar]

r? @marcoieni