Proposal

Add new index-level setting called index.rollover_date that will record the time an index was
rolled over.

Usage

1. general index management. It would be nice to know
2. it could be used by systems like Index-Lifecycle-Management to keep track of relative age of
   the documents within an index. For example, we would only want to move on to the warm phase after 7 days. Is this 7 days after the index was created, or rolled over? We believe that it should be
   relative to the rollover date. (see inherit [index.lifecycle.date] from rolled-over time #30853 (comment))

Implementation

Add an additional callback action to when the rollover aliases are appropriately swapped to also update index.rollover_date on the rolled over index, and then continue the active-shards check before returning the rollover response