Measure duration of time a label has been applied across an instance

Proposal

A customer reports a compliance need to measure the duration of calendar time a label has been applied across the instance. They use labels to denote blockers and add/remove them as they are uncovered and resolved. A metrics report with this information would allow them to, for instance, track how long a firewall problem was a blocker for a certain issue to proceed.

This feature would likely be contingent on related issue 433248.