Stats endpoints swagger states wrong return type

Some of the */stats endpoints have return types of DataPoint for the swagger annotation. But they actually return something else.

https://github.com/hawkular/hawkular-metrics/tree/master/api/metrics-api-jaxrs/src/main/java/org/hawkular/metrics/api/jaxrs/handler

https://github.com/hawkular/hawkular-metrics/pull/592