Database

[{"_id":ObjectId("63a994974ac549c5ea982d2b"),"title":"Destroyer","year":2018},{"_id":ObjectId("63a994974ac549c5ea982d2a"),"title":"Aquaman","year":2014},{"_id":ObjectId("63a994974ac549c5ea982d29"),"title":"On the Basis of Sex","year":1996},{"_id":ObjectId("63a994974ac549c5ea982d28"),"title":"Holmes and Watson","year":1940},{"_id":ObjectId("63a994974ac549c5ea982d27"),"title":"Conundrum: Secrets Among Friends","year":1957},{"_id":ObjectId("63a994974ac549c5ea982d26"),"title":"Welcome to Marwen","year":2004},{"_id":ObjectId("63a994974ac549c5ea982d25"),"title":"Mary Poppins Returns","year":1997},{"_id":ObjectId("63a994974ac549c5ea982d24"),"title":"Bumblebee","year":2004}]

Query

db.collection.aggregate([{$group:{"_id":{"year":"$year"},year:{$first:"$year"}}},{$addFields:{leap:{"$and":[{"$eq":[{"$mod":["$year",4]},0]},{"$or":[{"$ne":[{"$mod":["$year",100]},0]},{"$eq":[{"$mod":["$year",400]},0]}]}]}}},{$group:{"_id":"Leap","count":{$sum:{$cond:{if:{$eq:["$leap",true]},then:1,else:0}}}}}])

Result