Metadata is just a list of key-value pairs of string type.
This type of metadata is meant to be generated by Pantavisor or any of the running containers to expose internal information.
Check the list of default key-value pairs that Pantavisor sends.
This type of metadata is meant to be generated by the user so Pantavisor or any of the containers can consume it.
Check out the list of default user metadata consumed by Pantavisor.