Ceilometer currently supports a few different methods for data sourcing: polling the individual service APIs, listening for service-emitted notifications, and via the POST method of the Ceilometer API. The first two methods are well-suited for collecting data from services that have access to the shared messaging bus and have the advantage of already-developed publishing mechanisms, but for a completely independent source it's feasible that the API could be used, with the caveat that the end result would be similar to directly inserting to the Ceilometer DB.