Document Metrics

I think we need a documentation-page for all Metrics that we emit. We are expecting people to monitor those but it is sometimes very hard to figure out what they actually mean.

The issue names no files, tests, or entry points. Start by locating the definitions or emission points for FoundationDB's metrics, then determine how their meanings should be presented; done means a documentation page covers all emitted metrics clearly enough for monitoring.

Written by the indexing model from the issue text.