[{"_id":1,"items":[{"k":"A","v":1},{"k":"B","v":2}]},{"_id":2,"items":[{"k":"A","v":3},{"k":"B","v":4}]}]

db.collection.aggregate([{$unwind:"$items"},{$group:{_id:"$items.k",v:{$sum:"$items.v"}}},{$group:{_id:null,items:{$push:{k:"$_id",v:"$v"}}}}])