title: Mosquitto not publishing on SYS topics
body: <p>I am using mosquitto server as mqtt broker. I testing mosquitto for performance and I need to subscribe to $SYS hierarchy for some data like number of currently connected from <code>$SYS/broker/clients/active</code> topic. I have following mosquitto config file.</p>
  
  <pre><code># Place your local configuration in /etc/mosquitto/conf.d/
  #
  # A full description of the configuration file is at
  # /usr/share/doc/mosquitto/examples/mosquitto.conf.example
  
  pid_file /var/run/mosquitto.pid
  
  persistence true
  persistence_location /var/lib/mosquitto/
  
  log_dest file /var/log/mosquitto/mosquitto.log
  
  include_dir /etc/mosquitto/conf.d
  
  listener 1884
  protocol websockets
  
  listener 1883
  
  sys_interval 1
  
  max_connections -1
  </code></pre>
  
  <p>I am subscribing to <code>$SYS/broker/clients/active</code> topic like this</p>
  
  <pre><code>ubuntu@linux-box:/etc/mosquitto$ mosquitto_sub -d -t $SYS/broker/clients/active
  Client mosqsub/28715-ip-172-31 sending CONNECT
  Client mosqsub/28715-ip-172-31 received CONNACK
  Client mosqsub/28715-ip-172-31 sending SUBSCRIBE (Mid: 1, Topic: /broker/clients/active, QoS: 0)
  Client mosqsub/28715-ip-172-31 received SUBACK
  Subscribed (mid: 1): 0
  </code></pre>
  
  <p>The <code>sys_interval</code> in config file is 1, but I am not receiving any updates on above subscription. I also tried this subscribing to some alternative topics, but still not receiving anything. Mosquitto server is hosted on an AWS micro instance with linux operating system.</p>