[{"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([{$addFields:{data:{$map:{input:{$objectToArray:"$data"},in:{day:"$$this.k",sum:{$reduce:{input:{$objectToArray:"$$this.v"},initialValue:0,in:{$add:["$$this.v","$$value"]}}}}}}}},{$unwind:"$data"},{$replaceRoot:{newRoot:"$data"}}])