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

db.collection.aggregate([{$addFields:{metadata:{$objectToArray:"$metadata"}}},{$unwind:"$metadata"},{$group:{_id:"distinct",dist:{$addToSet:"$metadata.k"}}}])