[{_id:"5e13asdfasuh",animal:"dog",gender:"male",age:5,name:"mark",breed:"gold"},{_id:"5e13asdfasuhss",animal:"dog",name:"snow",age:"2",breed:"husky"}]

db.collection.aggregate([{$addFields:{gender_count:{$cond:[{"$ifNull":["$gender",false]},1,"$$REMOVE"]}}}])