[{"data":{"1":10,"2":30,}},{"data":{"2":10,"3":30,}}]

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