prometheus_exporters: Improve arch-textcollector

I use these metrics for my own infrastructure as well and find them to be quite useful. With pacnew you will know, when you need to upgrade a config file. I didn't add an alert yet because I thought that maybe a lot of files will have accumulated.

The second t for orphans is useful in case a PKGBUILD gets updated and a dependency moved to optdepends. That case could theoretically break a playbook when deploying a new machine.