I am integrating my app with the MQTT functionality of chirpstack, and really like the idea of variables that I can add to the devices. When reading the docs, it sounds like the variables and tags for a device should be automatically added to the MQTT messages? Is that true? They aren’t showing up on my messages on the application/<appid>/device/<deveui>/rx topic. Is there a way I can add the tags or variables to the MQTT integration?
Tags are exposed, variables not as they can be used to configure per integration security tokens (e.g. in case of ThingsBoard). You don’t want to expose this case of info over MQTT 
OK, but how do I access them. Like how does the the Thingsboard integration use them? Can I use them in the MQTT topic template?
Internally they are passed to the integration code (e.g. chirpstack-application-server/internal/integration/thingsboard/thingsboard.go at master · brocaar/chirpstack-application-server · GitHub). You can’t use them as an end-user, unless you integrate with the API directly.