https://stackoverflow.com/questions/6563885/socket-io-how-do-i-get-a-list-of-connected-sockets-clients
https://stackoverflow.com/questions/66966749/socket-io-v-4-how-to-get-list-of-sockets-already-in-room-when-joining
구조 파악 : https://github.com/Jeontaeyun/chat-back
