As-is, the metrics component assumes control of starting/stopping duration tracking, and the timestamps are opaque to the surrounding system. For recovery, the resharding machines need direct access to the timestamps to durably persist them to metadata documents. To solve this, we will need to either:

1. Change the metrics component so that starting/stopping tracking returns a timestamp, or
2. Invert responsibility so that the resharding state machines generate timestamps then pass them into the metrics component.