Collect pack-object metrics through Git Trace2

From #2769 (comment 1143842619):

How about we do a first iteration and focus on the metrics surfaced through the trace2 mechanism for pack-objects, which include the following list:

• how many pack objects are being written
• loose objects written
• number of objects in a packfile
• time in prepare-pack
• time writing packfile

It would be neat to get a sense of these numbers by throwing them into Prometheus metrics (maybe histograms or summaries), so we can start getting a sense of what Git is doing on our most resource heavy operations which tend to be git-pack-objects.

This way, we can:

• get some useful metrics around pack-objects, which we need in the near term anyway
• prove out the usefulness of trace2
• built out a foundation to more easily add instrumentation of trace2 on other git operations in Gitaly in the future