[{"key":1,"title":"Product A","attrs":[{"key":"size","value":"small"},{"key":"color","value":"red"}]},{"key":2,"title":"Product B","attrs":[{"key":"size","value":"large"},{"key":"color","value":"blue"}]},{"key":3,"title":"Product C","attrs":[{"key":"resolution","value":"8K"},{"key":"refresh rate","value":"60 Hz"},]}]

db.collection.aggregate([{"$unwind":"$attrs"},{"$group":{"_id":{k:"$attrs.key",v:"$attrs.value"},"count":{"$sum":1}}},{"$group":{"_id":"$_id.k","values":{"$push":{"title":"$_id.v","count":"$count"}}}},{"$project":{"_id":{key:"$_id",values:"$values"}}}])