[{"_id":{"month":4},"count":1.0},{"_id":{"month":5},"count":8.0},{"_id":{"month":2},"count":1.0},{"_id":{"month":3},"count":1.0}]

db.collection.aggregate([{$addFields:{"_id.month":{$switch:{branches:[{case:{$eq:["$_id.month",1]},then:"January"},{case:{$eq:["$_id.month",2]},then:"February"},{case:{$eq:["$_id.month",3]},then:"March"},{case:{$eq:["$_id.month",4]},then:"April"}],default:"December"}}}}])