metrics: Add `reference_backend` label to the git command metrics

First, we build on top of !7247 (commits) and do some more additional cleanups. The first two commits are in this direction.

Second, we add a new reference_backend label to the command metrics. This will help us collate and compare metrics for and during the migration from the files backend to the reftable backend. Unfortunately, we do not add any tests here, since the metrics are registered against the DefaultRegisterer and as such any tests which would be running in parallel would affect the outcomes of these metrcs.