[{"name":"name1","status":"one","unit":[],"child":[],"element":["6604"],"tags":["tag1","tag2","tag3"]},{"name":"name2","status":"one","unit":[],"child":[],"element":["6604"],"tags":["tag1","tag2","tag3","tag4"]}]

db.collection.aggregate([{$unwind:"$tags"},{$group:{_id:null,tags:{$addToSet:"$tags"}}},{$project:{_id:0,tags:{$sortArray:{input:"$tags",sortBy:1}}}}])