[{"id":1,"no":1,"val":"a"},{"id":2,"no":2,"val":"a"},{"id":3,"no":3,"val":"b"},{"id":4,"no":4,"val":"c"},]

db.collection.aggregate([{"$match":{}},{"$group":{"_id":"$val","count":{"$sum":1}}},{"$project":{"field":{"$arrayToObject":[[{k:{"$concat":["Value ","$_id"]},v:"$count"}]]}}},{"$replaceWith":"$field"}])