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

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