The Ceilometer API was deprecated at Pike and seems to have been removed since: https://docs.openstack.org/ceilometer.... In any case, it never allowed setting the granularity or requesting samples manually.

Ceilometer has two methods to get data. First from notifications emitted by the services that it measures. Ceilometer has no control over the frequency of those notifications, but perhaps you can tune them by setting service-specific parameters. it seems to me, though, that notification are emitted whenever some change occurs, so that frequency doesn't apply.

The second method is Ceilometer actively polling services. The polling frequency is set in a config file, not via API.