Database
[{"Name":"bob","Profession":"IT","Height":200},{"Name":"simon","Weight":100,"IQ":120},{"Name":"james","Weight":130,"Glasses":"Yes"}]
Query
db.collection.aggregate([{"$project":{"keys":{"$objectToArray":"$$ROOT"}}},{"$group":{"_id":null,"keys":{"$addToSet":"$keys.k"}}},{"$project":{"_id":0,"keys":{"$size":{"$reduce":{"input":"$keys","initialValue":[],"in":{"$setUnion":["$$value","$$this"]}}}}}}])