code: public void notifyNewMessage(Plugin plugin) {
          if (plugin == null) {
              throw new IllegalArgumentException("Parameter plugin must not be null.");
          }
  
          PluginStats pluginStats = mapPluginStats.get(plugin.getId());
          if (pluginStats != null) {
              pluginStats.incMessageCount();
          }
      }
docs: Notifies that the given {@code plugin} sent (and received) a non-HTTP message.
  <p>
  The call to this method has no effect if there's no {@code Plugin} with the given ID (or, it was not yet started).
  
  @param plugin the plugin that sent a non-HTTP message
  @throws IllegalArgumentException if the given parameter is {@code null}.
  @since 2.5.0
  @see #notifyNewMessage(Plugin, HttpMessage)