Developer productivity metrics chart dropped in 11.7

In the 11.7 retro, @meks mentioned that https://quality-dashboard.gitlap.com/groups/gitlab-org/developer_productivity_metrics went down. This measures MRs merged per author among the team at GitLab.

@tommy.morgan asked us (@DouweM, @lmcandrew, @marin, @rnienaber, and me) to look into why this might be the case for our teams, in particular around time off: how many days did the team take off during 11.7 work compared to 11.6 work?

We'll use this issue to answer that question and add some other thoughts on the way we measure and interpret this data.