Database
[{"_id":ObjectId("5d925bd3fabc692265f950d5"),"internalId":1,"first":{"second":[{"value":10}]}}]
Query
db.collection.aggregate([{"$addFields":{"first.second.tmpSize":{"$reduce":{"input":"$first.second","initialValue":"","in":{$strLenCP:{$ifNull:[{$toString:"$$this.value"},""]}}}}}}])