[{"date":"2019-11-04","count":39},{"date":"2019-11-05","count":135}]

db.collection.aggregate([{$group:{_id:null,total:{$sum:"$count"},docs:{$push:"$$ROOT"}}},{$project:{docs:{$map:{input:"$docs",in:{date:"$$this.date",count:"$$this.count",percentage:{$concat:[{$toString:{$round:{$multiply:[{$divide:["$$this.count","$total"]},100]}}},"%"]}}}}}},{$unwind:"$docs"},{$replaceRoot:{newRoot:"$docs"}}])