Database

[{"non_relevant_1":"ABC","items_array":{"item_1":1,"item_2":2,"item_3":1,"item_4":1}},{"non_relevant_1":"HIJ","items_array":{"item_1":3,"item_2":2,"item_3":4}}]

Query

db.collection.aggregate([{"$addFields":{"items_array":{"$map":{"input":{"$map":{"input":{"$objectToArray":"$items_array"},"as":"m","in":["$$m.k","$$m.v"]}},"as":"item","in":{"name":{"$arrayElemAt":["$$item",0]},"qty":{"$arrayElemAt":["$$item",1]}}}}}},{"$unwind":{"path":"$items_array"}},{"$group":{"_id":"$items_array.name","total-qty":{"$sum":"$items_array.qty"}}},{"$group":{"_id":null,"items_array":{"$push":{"$map":{"input":{"$map":{"input":{"$objectToArray":"$$ROOT"},"as":"m","in":["$$m.k","$$m.v"]}},"as":"i","in":{"$arrayElemAt":["$$i",1]}}}}}},{"$project":{"_id":0}},{"$addFields":{"items_array":{"$arrayToObject":"$items_array"}}}])

Result