@Override
        public String toString() {
            return "HeapHistogramEntry{" +
                    "className='" + className + '\'' +
                    ", instances=" + instances +
                    ", bytes=" + bytes +
                    '}';
        }
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