[{"city":["something","somethingElse"]},{"city":["somethingElse"]},{"country":"IDK"},{"city":["notMyCity"]},{"city":["whoKnows"]},{"city":["whoKnows","something"]},{"city":["something"]}]

db.collection.aggregate([{$unwind:"$city"},{$group:{_id:"$city",count:{$sum:1}}},{$group:{_id:"",citiesAndCount:{$push:{city:"$_id",count:"$count"}}}}])