gitserver: Add instrumentation for clone time

Created by: indradhanush

We currently have one way of cloning a repo: from an upstream URL. Very soon we will start using the CloneFromShard option to migrate a repo from one gitserver instance to another. We should have a metric to capture how long this takes. This will help us decide concurrency factor of the migration once its running. And since we will be instrumenting this, we might as well use a VectorCount to also instrument the regular clones.