Provide detailed documentation regarding metrics collection

[aboguszewski-sumo]

As someone who's never been very familiar with Prometheus, I often feel baffled by how our metrics collection solution works and I keep forgetting things about it. Since we're removing Prometheus in v5.0, I feel it would make lots of sense to create a doc that explains our metrics collection in detail.

Assigning this to myself, as I hope to have time to do this someday.