Document how to use our prometheus exporters

Hello! I keep seeing Snowflake maintainers post nice charts of Snowflake metrics (https://snowflake-broker.torproject.net/metrics, https://snowflake-broker.torproject.net/prometheus, and sometimes even some Snowflake server stats).

I was wondering if there is a public instance of Grafana or something where I can see them as well.

If not, I think it'd be nice to add some instructions or at least a pointer to how to set up one such site.

BTW, for proxies we also have metrics-address, but instructions as to what can be done with it are lacking.