Improve documentation for Prometheus metrics

For anyone who wants to use the exposed Prometheus metrics, it would be nice to have more details in the docs about these.

Specifically, it's helpful to know for each metrics:

- whether gauge or counter or something else
- units for data
- labels, and brief description of values the label could take

https://fly.io/docs/reference/metrics/#built-in-metrics is one example of such a doc. Our metrics are a little more complicated in that different metrics have very different labels.

cc @hendrikmakait