[Relay Operators] What's MetricsPort and how to enable it

As MetricsPort is a new Tor feature, we should add a new entry explaining what's it, how operators can enable it, and what precautions they should take.

We should not copy and paste little-t-tor docs, but point it as a reference.

   MetricsPort [address:]port [format]
       WARNING: Before enabling this, it is important to understand that exposing tor metrics publicly is dangerous to the Tor network users. Please take extra precaution and care when opening this
       port. Set a very strict access policy with MetricsPortPolicy and consider using your operating systems firewall features for defense in depth.

       We recommend, for the prometheus format, that the only address that can access this port should be the Prometheus server itself. Remember that the connection is unencrypted (HTTP) hence consider
       using a tool like stunnel to secure the link from this port to the server.

       If set, open this port to listen for an HTTP GET request to "/metrics". Upon a request, the collected metrics in the the tor instance are formatted for the given format and then sent back. If
       this is set, MetricsPortPolicy must be defined else every request will be rejected.

       Supported format is "prometheus" which is also the default if not set. The Prometheus data model can be found here: https://prometheus.io/docs/concepts/data_model/

       The tor metrics are constantly collected and they solely consists of counters. Thus, asking for those metrics is very lightweight on the tor process. (Default: None)

       As an example, here only 5.6.7.8 will be allowed to connect:

           MetricsPort 1.2.3.4:9035
           MetricsPortPolicy accept 5.6.7.8

   MetricsPortPolicy policy,policy,...
       Set an entrance policy for the MetricsPort, to limit who can access it. The policies have the same form as exit policies below, except that port specifiers are ignored. For multiple entries,
       this line can be used multiple times. It is a reject all by default policy. (Default: None)

       Please, keep in mind here that if the server collecting metrics on the MetricsPort is behind a NAT, then everything behind it can access it. This is similar for the case of allowing localhost,
       every users on the server will be able to access it. Again, strongly consider using a tool like stunnel to secure the link or to strengthen access control.