Comment 1 for bug 1773171

https://docs.openstack.org/releasenotes/ceilometer/ocata.html#prelude

"In an effort to minimise the noise, Ceilometer will no longer produce meters which have no measureable data associated with it. Image meter only captures state information which is already captured in events and other meters."

Ceilometer used to convert meters to events but does not do so as of:

https://github.com/openstack/ceilometer/commit/2d31c249f9bf28248773fe080650dbfa8be7e824#diff-ed385302763b1b83d441c58ff98aa647L16

Some meters had semantic meaning of events as far as I recall and that got fixed in some version after Mitaka.