Feature-request: official Grafana dashboard

## What?
Graylog should add an official grafana dashboard matching the prometheus exporter metrics with some reasonable defaults to check. Other companies/projects like Kong, CoreDNS and so on already pushed dashboards to Grafana. I assume, that with the increasing amount of k8s instances one would like to have single source for visualizations of their environment

## Why?
Unfortunately, there's no documentation of the exact meaning of each metric available (see https://github.com/Graylog2/graylog2-server/issues/13169), so it's very hard to create an advanced grafana dashboard on your own.

There's a quite old dashboard available: https://gist.github.com/asachs01/973ca544d9e81861e6fae22013d88703 which might be a good starting point.