[{"count":{"a":1,"b":5,"c":7}},{"count":{"a":4,"c":2,"d":6}}]

db.collection.aggregate([{$set:{count:{"$objectToArray":"$count"}}},{$unwind:"$count"},{$group:{_id:"$count.k",v:{$sum:"$count.v"}}},{$project:{v:1,k:"$_id",_id:0}},{$group:{_id:"total",count:{$push:{k:"$k",v:"$v"}}}},{$project:{_id:0,count:{"$arrayToObject":"$count"}}}])