[{"car_cost":1,"car_mileage":5},{"car_cost":2,"car_mileage":6},{"car_cost":3,"car_mileage":7}]

db.collection.aggregate([{"$group":{"_id":null,"avg_cost":{"$avg":"$car_cost"},"min_cost":{"$min":"$car_cost"},"max_cost":{"$max":"$car_cost"},"avg_mileage":{"$avg":"$car_mileage"},"min_mileage":{"$min":"$car_mileage"},"max_mileage":{"$max":"$car_mileage"}}},{"$set":{"array":[{"metric":"cost","avg":"$avg_cost","min":"$min_cost","max":"$max_cost"},{"metric":"mileage","avg":"$avg_mileage","min":"$min_mileage","max":"$max_mileage"}]}},{"$unwind":{"path":"$array"}},{"$replaceRoot":{"newRoot":"$array"}}])