Database
[{"a":10,"b":20,"c":30},{"a":11,"b":20,"c":31},{"a":10,"b":20,"c":31}]
Query
db.collection.aggregate([{"$project":{_id:0,arr:{"$objectToArray":"$$ROOT"}}},{"$unwind":"$arr"},{$match:{"arr.k":{$ne:"_id"}}},{$group:{_id:"$arr.k",values:{"$addToSet":"$arr.v"}}}])