I am working on an iot device using google-cloud-iot-core, but i would like to have allow for 3rd party support, so i want to change the host name of the mqtt, how can it be done?
Thanks in advance
If you change that endpoint, then you're no longer talking to IoT Core. It's not configurable, as that's the endpoint to talk to the service.
You can setup your own MQTT server somewhere else (could do it in GCE or GKE with a custom container) and make the hostname whatever you want, and then setup your own broker to take the MQTT payloads and create Pub/Sub messages, or even to act as a forwarding proxy to IoT Core itself I suppose (although security and auth might get a bit odd).
Or you could even just go directly to Pub/Sub. It all just depends on your need.
As I mentioned, changing the hostname for IoT Core (the mqtt.googleapis.com) it means that you aren't using IoT Core any longer. There's no other way to access the communication broker piece of IoT Core to do the Pub/Sub message creation, etc. If you don't use the IoT Core endpoint (hostname) then you'd be on your own for creating the Pub/Sub messages from the IoT device data.