[{"_id":1,"createdAt":ISODate("2024-02-14T01:02:03Z"),"viewCount":1,"clickCount":1},{"_id":2,"createdAt":ISODate("2024-02-14T01:03:03Z"),"viewCount":2,"clickCount":2},{"_id":3,"createdAt":ISODate("2024-02-16T01:02:03Z"),"viewCount":4,"clickCount":4}]

db.collection.aggregate([{$group:{_id:{$dateTrunc:{date:"$createdAt",unit:"day"}},viewCount:{$sum:"$viewCount"},clickCount:{$sum:"$clickCount"}}},{$sort:{_id:1}},{$densify:{field:"_id",range:{step:1,bounds:"full",unit:"day"}}},{$fill:{output:{viewCount:{value:0},clickCount:{value:0}}}}])