Usage Ping data for Merge Requests

Problem to solve

groupsource code will be using Percentage of users per instance using MRs as its north star metric, but this data is not currently in the usage ping.

This issue is here in order to reference the SSOT (single source of truth) issue: https://gitlab.com/gitlab-data/analytics/-/issues/4633

This issue will serve as weighting and as an assignment for work to begin, but all other investigations and actions should be done in the original issue.