[{_id:"45454",name:"fulano",city:"cali",data:{age:12,lastName:"panguano",cars:0}},{_id:"67899",name:"juanito",city:"cali",data:{age:23,lastName:"merlano",cars:2}},{_id:"555",name:"olito",city:"nw",data:{lastName:"betito",cars:2}},{_id:"11223",name:"cabrito",city:"trujillo",data:{age:28,cars:1,moto:3}},]

db.collection.aggregate([{$project:{keys:{$map:{input:{"$objectToArray":"$data"},in:"$$this.k"}}}},{"$unwind":"$keys"},{$group:{_id:"$keys"}}])