Don't make Prometheus monitor prometheus itself

[geerlingguy]

By default, this configuration monitors Prometheus, and scrapes every 5 seconds. There are like 1,000 metrics Prometheus exports, so that's a lot of data to be scraping 720 times per hour, or over 17,000 times per day, especially since none of it is exposed in the dashboards we ship with this project.

So, I propose adding a new prometheus_monitor_prometheus variable, and defaulting it to false.

Not only do I propose it, I'm about to push the variable to the project, and then this will be done, heh.

[geerlingguy]

I also noticed a previous PR broke the spacing for the pihole-exporter config, so I'm going to fix that here.