Add unique users using MRs to usage ping
Problem to solve
groupsource code will use percentage of users making use of MRs per instance as our north star metric, however, this is not currently instrumented.
Further details
This metric will be supported by the secondary metric average MR duration
https://about.gitlab.com/handbook/product/feature-instrumentation/
Proposal
Add unique users making use of MRs to the usage ping. Once this is added, we should be able to compare the total number of instance users vs. the new metric to get the percentage. If we don't have "total instance users" we'll need that too.