title: How to get a list of all topics containing specific values known to MQTT broker?
body: <p><br>
  I'm looking for a way to get a list of all topics known to a broker. There are some quite similar question's, but they didn't help me to figure it out for my use case. <br> 
  I've got 3 Raspberry Pi's with multiple sensors (temperature, humidity) which are connected over an MQTT network. Every Pi has it's own database containing time series of measurements and other system variables(like CPU). <br> 
  Now I'm looking for a way for the following szenario: <br></p>
  
  <p>I want to monitor my system and detect anomalies. For that I want to get all sensor-time series in the last x seconds and process them in a python script. My system to do the monitoring calculations can be every Pi. <br> <br>
  <strong>Example:</strong> I'm on RPI2 and want to monitor the whole distributed network. There's no given knowledge about the sensors attached to the Pi's. Now from my python script running on RP2 I would initalise a MQTT client and subscribe every sensor data on the broker. <br> I know about the wildcard # but I'm not sure how to use it in that case. My magic command would look like the following pseudo code: <br> </p>
  
  <pre><code>1) client subscribe to all sensor data - #/sensor/#
  2) get list with all topics 
  3) client subscribe to all topics from given list list/#
  4) analyse data for anomalies every x seconds
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/VpVme.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VpVme.png" alt="Distributed Network with broker and sensors"></a></p>