PrometheusReportingTask does not report nested process group status

The clear() command added in NIFI-6715 causes the registry to be reset on every call to createNiFiMetrics. In the case of nested process groups this method is a recursive call, so the registry keeps getting cleared except for the last visited process group.

The clear() should only be done when createNiFiMetrics is called on the root process group, in order to clean up the registry before adding all the components' metrics.