[{stats:{histogram:[{"duration":1208,"dataPoints":2,"binMin":0,"binMax":5000},{"duration":25735,"dataPoints":3,"binMin":5000,"binMax":10000},{"duration":0,"dataPoints":0,"binMin":10000,"binMax":20000},{"duration":54088,"dataPoints":2,"binMin":20000,"binMax":28817}]}}]

db.collection.aggregate([{"$addFields":{"totalDuration":{"$reduce":{"input":"$stats.histogram","initialValue":0,"in":{$add:["$$value","$$this.duration"]}}}}},{"$addFields":{"totalDuration":{"$cond":{"if":{$eq:["$totalDuration",0]},"then":1,"else":"$totalDuration"}}}},{"$addFields":{"stats.histogram":{"$map":{"input":"$stats.histogram","as":"h","in":{"duration":"$$h.duration","dataPoints":"$$h.dataPoints","binMin":"$$h.binMin","binMax":"$$h.binMax","ratio":{"$divide":["$$h.duration","$totalDuration"]}}}}}}])