Gateway Prometheus plugin doc rework

## Definition of done
The Prometheus plugin doc is outdated and incorrect. Work with @sichvoge to bring this up to date.

We need to:
* rework the [reference/landing page for Prometheus](https://developer.konghq.com/plugins/prometheus/)
* add a use case table and a how it works section
* create a bunch of new examples to cover the use cases mentioned in the draft doc

## Information
Work with Christian.

## Size
L

Start with the Prometheus plugin reference landing page linked in the issue and review the current documentation against the draft. Done means the page is accurate, includes a use case table and how it works section, and has new examples covering the documented use cases; coordinate with Christian as requested.

Written by the indexing model from the issue text.