[{"id":"id1","tags":["a","b"]},{"id":"id2","tags":["b","c"]},{"id":"id3","tags":["a","c"]}]

db.collection.aggregate([{"$unwind":"$tags"},{"$group":{"_id":"$tags","count":{"$sum":1}}},{"$replaceRoot":{"newRoot":{"$arrayToObject":[[{"k":"$_id","v":"$count"}]]}}}])