Feature request: Export metrics to Prometheus

1. What's the feature?

• Export metrics to Prometheus on:
  • Scrap iptables data to get network information (blocked data, vpn'ed data etc.)
  • Health information, such as when connectivity drops
  • VPN server information
  • the Go written Shadowsocks
  • the (Go) HTTP proxy
  • the Go written DNS server (blocked by #137)
  • the numbers found from #251
  • Maybe add some bandwidth tests but that depends on the test server, so maybe not
• Add instructions on how to set this up with Grafana to have nice graphs See https://github.com/qdm12/gluetun-wiki/blob/latest/setup/advanced/metrics.md
• Add a Grafana dashboard example to the repository See https://github.com/passteque/gluetun/blob/master/metrics/grafana.json

1. Why do you need this feature?

Because metrics are fun

1. Extra information?