Replace custom statistics by Prometheus metrics

[maringuu]

First, let me say that the choice of Prometheus is arbitrary and only based on the fact that I have heard of it before and
seen it used in other software I use.
Thus, the choice of Progmeheus should be evaluated before implementing this proposal.

Currently, there are two types of server usage statistics on the /system_health endpoint.

1. CPU usage, disk space, distribution, python version, and FACT version.
2. The lengths of queues below for the individual plugins.

The first one can trivially be replaced by Prometheus.
Displaying distribution and python version is not needed imo, but is definitely possible (but not in the demo link).

The second one is a bit more involved but can also be done by writing a Prometheus client library.

Further, I would argue that both statistics should not be exposed to the user but rather only to the admin hosting FACT.
Removing them would trivially solve this.

Closes: fkie-cad/FACT_docker#45
Related: #991