Write down guidelines for adding new stats

We're going to add a few new stats to Tor Metrics in the next months: BridgeDB, PrivCount, sbws, and maybe more, in no specific order.

Let's write down some initial guidelines what we expect and what others can expect. And let's refine these initial guidelines as we add some actual stats. Once we're happy with them we should put them on Tor Metrics.

Assigning this ticket to myself for now, as I'm going to post a first draft soon. Cc'ing a few folks who we'll be working together with on adding new data.