Add time to issue epic relationship tracking
Per discussion in https://gitlab.com/gitlab-data/analytics/-/issues/5625#note_387835970
In order to begin tracking the number of issues assigned to epics, over time, we need to track time or status.
The end goal of https://gitlab.com/gitlab-data/analytics/-/issues/5625 is to be able to see a chart that shows the total number of issues that have an epic parent, over time, to track and understand adoption.
Update: as discussed bellow, we could start with adding EpicIssue count to usage data, which should give us similar numbers (sufficient for tracking adoption).
Edited by Jan Provaznik