[{"subject_id":1,"gender":"Male"},{"subject_id":1,"gender":"Male"},{"subject_id":1,"gender":"Male"},{"subject_id":2,"gender":"Female"},{"subject_id":3,"gender":"Female"},{"subject_id":4,"gender":"Unknown"}]

db.collection.aggregate([{"$group":{"_id":"$subject_id","gender":{"$first":"$gender"}}},{"$facet":{"nDocs":[{"$count":"nDocs"},],"groupValues":[{"$group":{"_id":"$gender","total":{"$sum":1}}},]}},{"$addFields":{"nDocs":{"$arrayElemAt":["$nDocs",0]}}},{"$unwind":"$groupValues"},{"$project":{"_id":0,"gender":"$groupValues._id","total":"$groupValues.total","percentage":{"$multiply":[{"$divide":["$groupValues.total","$nDocs.nDocs"]},100]}}}])