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"}}},{"$unwind":"$keys"},{"$match":{"keys.k":{"$ne":"_id"}}},{"$group":{"_id":"$keys.k","total":{"$sum":1}}},{"$group":{"_id":null,"total":{"$sum":1}}}])