title: How to find all users subscribed to a topic in spring websockets
body: <p>When making something like a chat application using Spring Websockets, it is useful to know who all is subscribed to any given topic.  For, e.g. presence data displayed in the clients.</p>
  
  <p>I know that I can implement ApplicationListener and keep my own list of "connected users", but it seems like the library must already be doing this work.  </p>
  
  <p>What's the recommended way to get active subscription info from the library directly (and without maintaining my own list in memory or db).</p>