[{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"},{$group:{_id:null,High:{$sum:{$cond:[{$eq:["$metadata.title","High"]},"$metadata.val",0]}},Medium:{$sum:{$cond:[{$eq:["$metadata.title","Medium"]},"$metadata.val",0]}},Low:{$sum:{$cond:[{$eq:["$metadata.title","Low"]},"$metadata.val",0]}}}}])