BP-36: Stats documentation annotation

**BP**

This is the master ticket for tracking BP-36 :

A common ask from people using bookkeeper is how they can monitor bookies and bookkeeper clients, what kind of metrics that bookkeeper exposes
and what are the important metrics. Currently bookkeeper doesn't provide any metrics page for guiding people on monitoring bookkeeper services.

In order to help people on this, we need to provide a few documentation pages about metrics. However if we just write such pages, those pages
can quickly get out-of-dated when code is changed. The proposal here is to seek a programming way for generating metrics related pages.

Proposal PR - #1786