VSD: Add "MR Rate" to the "Metrics comparative" widget

Problem to solve

Engineering managers need more visibility into the the team productivity and efficiency.

This is Dogfooding feedback.

Proposal

1. Adding a new metric "MR Rate" - counts of the total number of MRs merged / count of the total number of contributors.
2. "MR Rate" will be calculated monthly, for this MVC no need for real-time results.
3. Adding periodical DB job to aggregates the data monthly.
4. Both for groups and projects.
5. Add drill-down from the metric to MR Analytics.
6. Once VSD - Comparison panel - [FE] adding panel-leve... (#388890 - closed) will be completed users can filter the "MR Rate" with labels.
7. GitLab Ultimate