[{"_id":1,"date":ISODate("2022-02-04T00:00:00Z")},{"_id":2,"date":ISODate("2022-02-04T00:00:00Z")},{"_id":3,"date":ISODate("2022-02-05T00:00:00Z")},{"_id":4,"date":ISODate("2022-03-04T00:00:00Z")}]

db.collection.aggregate([{$group:{_id:{$dateTrunc:{date:"$date",unit:"month"}},cnt:{$sum:1}}},{"$setWindowFields":{"partitionBy":null,"sortBy":{"_id":1},"output":{"prev":{$first:"$cnt",window:{documents:[-1,-1]}}}}},{$set:{delta:{$subtract:["$cnt","$prev"]}}},{$unset:"prev"},{$merge:{into:"sales_by_month",whenMatched:"discard",whenNotMatched:"insert"}}])