prometheus_exporters: Add total number of packages per directory to exporter

Sven-Hendrik Haaserequested to merge
add-packages-total-to-exporter into master

The idea of this is to allow us to graph the number of packages per repository in Grafana just like we're already doing in the archive exporter. I will create a follow-up that adds that functionality to the dashboard after this is merged and deployed.

Edited by Sven-Hendrik Haase

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