gitserver: some exec prometheus metric improvements

Created by: keegancsmith

We have inconsistent labeling in the prometheus metrics for gitserver exec. When investigating who runs exec the two fields that would be useful to record are:

• the git cmd being run (eg archive vs show)
• the exit code and/or success
• the client name (eg repo-updater or frontend)

We should also consider giving a more granular "client" name for all the different consumers in frontend. This would help us differentiate between language stats vs highlighting vs search / etc.

cc @sourcegraph/distribution who may also be interested in improving this for debugging customer issues.