Record creation date in Metastore

To make time-based rotation logic easier, add a creation timestamp in Metastore. Ensure that backwards compatibility is maintained. See about building time-based rotation into request logic so consumers get it automatically.

Start by locating Metastore and the request logic, then trace how existing metadata is stored and read. Define how a creation timestamp can be added without breaking older data, and confirm that time-based rotation information reaches consumers automatically.

Written by the indexing model from the issue text.