Get times labels were applied to and removed from issues

Proposal

A customer reports a compliance need to report all times a label was applied to and removed from issues. They use labels to denote blockers and add/remove the, as they are uncovered and resolved. This would allow them to track when, for instance, a firewall problem was a blocker, and when it was resolved. This data could be made available as metrics in the Admin area > Analytics page.

This issue would likely be a prerequisite for related issue 433254.