This exporter translates from MQTT topics to prometheus metrics.
The core design is that clients send arbitrary JSON messages on the topics.
The translation between the MQTT representation and prometheus metrics is
configured in the mqtt2prometheus exporter since we often can not change
the IoT devices sending the messages.
Clients can push metrics via MQTT to an MQTT Broker.
This exporter subscribes to the broker and expose the received messages
as prometheus metrics.
Currently, the exporter supports only MQTT 3.1.