[{"data":{"11-10-2001":{"a":17.281150000000001,"b":11.864060000000006},"13-10-2020":{"b":2.7616699999999994},"12-10-2001":{"b":4.0809599999999997},"09-10-2001":{"b":4.1286300000000005},"17-10-2001":{"a":15.140560000000123,"b":5.017139999999998},"18-10-2001":{"b":1.975189999999997,"a":7.093789999999976}}}]

db.collection.aggregate([{$project:{data:{"$objectToArray":"$data"}}},{$unwind:"$data"},{"$replaceRoot":{"newRoot":"$data"}},{$addFields:{v:{"$objectToArray":"$v"}}},{$addFields:{v:{$reduce:{input:"$v",initialValue:0,in:{$add:["$$this.v","$$value"]}}}}},{$group:{_id:null,data:{$push:{day:"$k",sum:"$v"}}}}])