process-exporter prometheus metrics should include git-pack-objects

We have interesting process statistics in prometheus under the namedprocess_ namespace created by process-exporter. These include several interesting processes on Gitaly services but they exclude git-pack-objects which is something we'd like to have data about for #688 (closed).

I would like to see git-pack-objects in this list: https://thanos-query.ops.gitlab.net/graph?g0.range_input=1h&g0.max_source_resolution=0s&g0.expr=sum(namedprocess_namegroup_cpu_seconds_total%7Btype%3D%22gitaly%22%7D)%20by%20(groupname)&g0.tab=1

The problem appears to be an incorrect regex in our gitlab-exporters cookbook. Some git processes are invoked via /opt/gitlab/embedded/bin/git and those get picked up by our cookbook. Others, such as pack-objects, are invoked via /opt/gitlab/embedded/libexec/git-core/git and those get overlooked. We should change the recipe so it no longer overlooks those processes.