Document the prometheus metrics that are being exported

[wileyj]

https://github.com/stacks-network/stacks-blockchain/blob/master/src/monitoring/mod.rs

It would be helpful to have a doc of what metrics are being exported, and what the values are shown in

[jcnelson]

Is there a way to do this via some sort of doc-generation software, so that you could just keep the official docs as code comments?

[wileyj]

not stale