[ceilometer] Add Ceph disk usage for images and instances

Hello, I've seen that it's possible to add polsters to ceilometer to gather additional metrics. I need to measure storage usage of my users, specially image usage (instance snapshots), where we still cannot have quotas. Does someone have already done something similar, where can I start to investigate ? I've seen some documentation to add Ceph RadosGW usage to Ceilometer, but it's not adaptable to my use case.