tavasti, right sorry I skimmed over the question. so same page. so on a server or instance the property is metadata so you can use the "set_server_metadata(server, **metadata)" in a python application.

import openstack
cloud = openstack.connect(cloud="openstack")
server = cloud.compute.get_server("my_instance_uuid")
 cloud.compute.set_server_metadata(server, foo="bar")