add metrics docs

[pwright]

@c-kruse I'm not sure whether all these are valid, looking at https://docs.google.com/spreadsheets/d/1A2Tzo7BGF_ET1U5FGFMgnrcFz0qqNcljjA5Hmlsni3w/edit?gid=0#gid=0

[c-kruse]

Thank you @pwright! Just a few suggestions with some context around the metrics I think we had aught to sweep under a rug.

[c-kruse]

Thank you for this Paul. Looks great!

One last call-out for posterity in case someone comes playing detective for why they can't access the metrics endpoint.