Docs: metrics documentation improvements

[barchetta]

Problem Description

Feedback from a customer

1. Improve description of various metrics. While duplicating existing content to provide more details about the different metrics might not be the best strategy, docs could link to source that provide more details.
2. In some cases, some summarization might also be useful and save significant headaches to the users. Take for example the Prometheus-specific suffixes: that info is scattered across multiple sources and quite difficult to reconcile.