[{_id:"2019-06",revenue:20391.71},{_id:"2019-03",revenue:10190.44}]

db.collection.aggregate([{"$group":{_id:null,data:{$push:{"k":"$_id","v":"$revenue"}}}},{"$project":{"res":{"$arrayToObject":"$data"},"_id":0}},{"$replaceWith":"$res"}])