Last modified timestamp

Last modified timestamp automatic property
This is probably a niche request and would in my opinion be turn on-off in the metadata_config.

{"indexed": ["example_metadata_field"], "track_last_modified": true (default false)} 

Last modified timestamp would be an automatic metadata field that would be set on upsert/update of an index and would track the last time (as per name) this vector had its metadata (or values) added/modified. Field should be indexed and thus searchable. Default false would provide this feature to the users that check and update their data regularly to keep up to date information maybe even track changes in documents over time.