Expose Prometheus metrics for shallow, partial and initial clones

This MR implements Prometheus counters that track shallow, partial and initial clones for both SSH and smarthttp services.

Note that it would probably make sense to unify both update-ref analysis functions. I have already done a similar thing in !1894 (1102748f), but that has only landed a few minutes ago. So I'll probably revisit this MR to make use of it. Edit: done.

References #1823 (closed)