[{_id:"2313123123",metadata:{path:"...",value:"...",name:"..."}},{_id:"23131231234",metadata:{path:"...",name:"...",origin:"...",}},{_id:"23131231235",metadata:{path:"...",source:"..."}},]

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