Reporting and being able to communicate one's value as a freelancer is so important. You need a freelancer who will explain the results of their work, how and why something is or is not working, and what they plan to do next. This is a big reason why you hire a freelancer in the first place - so you don't have to take the time to determine these things.

Not every freelancer has access to metrics for all jobs, so don't hold it against them if they haven't had the opportunity to do this. But if not, ask them how they would do it.