[{_id:1,spec:{"abc":"abc","bcd":"bcd","cde":"cde"}},{_id:2,spec:{"bcd":"bcd","cde":"cde","efg":"efg"}},{_id:3,spec:{"cde":"cde","efg":"efg","ghi":"ghi"}},{_id:4,spec:{"ghi":"ghi","abc":"abc","bcd":"bcd"}}]

db.collection.aggregate([{$project:{spec:{$objectToArray:"$spec"}}},{$unwind:"$spec"},{$group:{_id:null,spec:{$addToSet:"$spec.k"}}}])