Add chart for average MRs merged per author per month

Throughput graphs are implemented at the weekly level and soon to be added at the monthly level.

Until we have good hygiene on milestones, tracking data using time periods: week/month in the span of the year is the most accurate.

This will be similar to what we have in https://quality-dashboard.gitlap.com/groups/gitlab-org/average_mrs_per_milestone but instead of milestones it will be per month.