Stop using type label for gitaly_pack_objects_* metrics

Overview

Each gitaly_pack_objects_* metric such as gitaly_pack_objects_queued expose a label called type

source

Unfortunately type is a revered label and usually means the name of the service, in our case type="gitaly"

This is dashboards like Gitaly pack-objects queued commands (gauage) isn't returning any data because it filters for type="gitaly"

source