[{metadata:[{"title":"High","val":12},{"title":"Medium","val":15},{"title":"Low","val":2}]},{metadata:[{"title":"High","val":10},{"title":"Medium","val":12},{"title":"Low","val":20}]}]

db.collection.aggregate([{"$unwind":"$metadata"},{$group:{_id:"$metadata.title",sum:{$sum:"$metadata.val"}}},{$group:{_id:null,metadata:{$mergeObjects:{$arrayToObject:[[{k:"$_id",v:"$sum"}]]}}}}])