[{a:1,b:2,c:3},{a:5,b:6,c:7}]

db.collection.aggregate({"$project":{"_id":0}},{"$project":{"arr":{"$objectToArray":"$$ROOT"}}},{"$unwind":"$arr"},{"$group":{"_id":"$arr.k","field":{"$push":"$arr.v"}}},{$replaceRoot:{newRoot:{$arrayToObject:[[{k:"$_id",v:"$field"}]]}}})