[{"key":1},{"key":2}]

db.collection.aggregate([{$project:{arrayofkeyvalue:{$objectToArray:"$$ROOT"}}},{$unwind:"$arrayofkeyvalue"},{$group:{_id:"$arrayofkeyvalue.k",value:{$first:"$arrayofkeyvalue.v"}}},{$group:{_id:null,allkeysandvalues:{$push:{k:"$_id",v:"$value"}}}},{$project:{allkeysandvalues:{$arrayToObject:"$allkeysandvalues"}}}])