[{"field_name":"field1","count":10},{"field_name":"field2","count":20},{"field_name":"field3","count":30},{"field_name":"field4","count":40}]

db.collection.aggregate([{"$facet":{"originalArr":[{$match:{count:{"$exists":true}}},{"$project":{_id:0}}],"totalArr":[{"$group":{"_id":null,"field_name":{"$first":"total"},"count":{$sum:"$count"}}},{"$project":{_id:0}}]}},{$project:{combined:{"$concatArrays":["$originalArr","$totalArr"]}}},{"$unwind":"$combined"},{"$replaceRoot":{"newRoot":"$combined"}}])