Add graylog.conf options to prometheus endpoint as gauges

## What?
In order to better monitor the deployed state of the application it is helpful to have metrics exposed that reflect the current configuration.

## Why?
Some settings are critical for performance or availability and might be different depending on the size of the cluster.
It is helpful to know what they are, and whether they changed over time, especially when investigating an issue.
This information is readily available and should be exposed as gauges through the prometheus endpoint.

Care needs to be taken to not leak credentials or other sensitive information, so we cannot simply expose everything blindly.

## Your Environment

* Graylog Version: 5.0.0-beta.3