Add doc page explaining metrics, close #224.

This adds a page that covers the _counters_, though not the timers; the original bug suggests discussing timers as well, but on IRC today it sounded like only counters were wanted. In any case, this moves us from "no metrics docs" to "metrics docs". I'm happy to add more as desired.

There's a fair amount of copy-and-paste; I tried to ensure two things happened: every metric gets mentioned by name at least once, so it can be googled, and every metric gets _either_ a paragraph explaining it directly (even if it's similar to some other paragraph) or a direct "exactly the same meaning as this other metric, just on a different API endpoint" sort of statement.

I can try to de-duplicate / factor further, but good docs are often a bit redundant in order to make the reader's life easier. They don't want to read more than necessary. Hopefully this was the right balance.