[{"episode":5,"plays":4,"dropoff":3737.25,"sessions":["2020-11-15","2020-11-15","2020-11-16","2020-11-15"],"events":4}]

db.collection.aggregate([/** your previous stages...*/{"$addFields":{"sessions":{"$map":{"input":"$sessions","in":{"k":"$$this","v":1}}}}},{"$unwind":"$sessions"},{$group:{_id:{_id:"$_id",dropoff:"$dropoff",episode:"$episode",events:"$events",plays:"$plays",date:"$sessions.k"},count:{$sum:"$sessions.v"}}},{$group:{_id:{_id:"$_id._id",dropoff:"$_id.dropoff",episode:"$_id.episode",events:"$_id.events",plays:"$_id.plays"},sessions:{$push:{k:"$_id.date",v:"$count"}}}},{"$project":{_id:"$_id._id",dropoff:"$_id.dropoff",episode:"$_id.episode",events:"$_id.events",plays:"$_id.plays",sessions:{"$arrayToObject":"$sessions"}}}])