title: Paho-MQTT check message queue size
body: <p>I'm publishing MQTT messages from an Arduino, and subscribing to those from a Raspberry Pi. Sometimes the publishing goes faster than the Raspberry can receive (and process).</p>
  
  <p>I'm looking for a way of checking how many messages are queued on the Raspberry side. I'm using <a href="https://pypi.org/project/paho-mqtt/" rel="nofollow noreferrer">Paho-MQTT</a>. I only see it is possible to set a max queue size, but how can I check the current queue size? (If possible.)</p>