@Override
        public int hashCode() {
            int result = className.hashCode();
            result = 31 * result + (int) (instances ^ (instances >>> 32));
            result = 31 * result + (int) (bytes ^ (bytes >>> 32));
            return result;
        }
Generates a heap histogram using the 'gcClassHistogram' MBean action.  Will
block until the action completes/fails.
list of {@link HeapHistogramEntry}
stroom-node/stroom-node-impl/src/main/java/stroom/node/impl/HeapHistogramService.java