[{"_id":ObjectId("5e4828297f8bc30a7515a85c"),"os_name":"x","os_version":1,"events":{"login":1,"last-event":"yesterday"},"type":"os"},{"_id":ObjectId("5e4828357f8bc30a7515acee"),"os_name":"x","os_version":1,"events":{"login":1,"last-event":"now"},"type":"os"},{"_id":ObjectId("5e4828437f8bc30a7515b17b"),"os_name":"x","os_version":2,"events":{"login":1,"last-event":"yesterday"},"type":"os"},{"_id":ObjectId("5e4828557f8bc30a7515b6a4"),"os_name":"y","os_version":1,"events":{"login":10,"last-event":"10 days back"},"type":"os"}]

db.collection.aggregate([{$group:{_id:{os_name:"$os_name",os_version:"$os_version"},"login":{$sum:"$events.login"},"events":{$last:"$events"},count:{$sum:NumberInt(1)}}},{$addFields:{"events.login":"$login"}},{$project:{login:0}}])