db={"outcoll":[{"_id":1,"timestamp":ISODate("2024-02-15T00:00:00Z")},{"_id":2,"timestamp":ISODate("2024-02-15T00:00:00Z")},{"_id":3,"timestamp":ISODate("2024-02-16T00:00:00Z")},{"_id":4,"timestamp":ISODate("2024-02-16T00:00:00Z")}],"counters":[{collection:"outcoll",seq:1}]}

db.outcoll.aggregate([{"$group":{"_id":null,"seq":{"$max":"$_id"}}},{"$project":{_id:0,collection:"outcoll",seq:1}},{"$merge":{"into":"counters","on":"collection","whenMatched":"merge"}}])