[{"_id":1,"os_name":"android"},{"_id":2,"os_name":"android"},{"_id":3,"os_name":"ios"},{"_id":4,"os_name":"ios"},{"_id":5,"os_name":"android"},{"_id":6,"os_name":"android"}]

db.collection.aggregate([{$group:{_id:null,by_os_ios:{$sum:{$cond:[{$eq:["$os_name","ios"]},1,0]}},by_os_android:{$sum:{$cond:[{$eq:["$os_name","android"]},1,0]}},total:{$sum:1}}},{$addFields:{"by_os.ios":"$by_os_ios","by_os.android":"$by_os_android"}},{$project:{_id:0,by_os_ios:0,by_os_android:0}}])