[dev][ceilometer] Add custom uptime metric to Ceilometer

Guys, I deployed Ceilometer into my OpenStack deployment using the Juju charm. I’m trying to add a custom metric that records the uptime of instances every hour. I’m totally lost on how to archive this or if it’s possible. Any help would be appreciate. So far I tried playing around with the meters.yaml and polling.yaml to see if I could pull it off with no luck. Is this possible? how should I do it? Thanks in advance. Regards, Ángel