[{"_id":ObjectId("5f3b2626927b18001db86884"),"collections":["Art","Craft"]},{"_id":ObjectId("5f3b2626927b18001db86885"),"collections":["Craft"]},{"_id":ObjectId("5f3b2626927b18001db86886"),"collections":["Apex","Art"]},{"_id":ObjectId("5f3b2626927b18001db86887"),"collections":[]}]

db.collection.aggregate([{$unwind:"$collections"},{"$group":{"_id":"$collections","v":{"$sum":1}}},{"$group":{"_id":null,"collections":{"$push":{$arrayToObject:[[{"k":"$$ROOT._id","v":"$$ROOT.v"}]]}}}},{$replaceRoot:{newRoot:{$mergeObjects:"$collections"}}}])