Can you add here something like:

  Asserts.succeedsEventually(new Runnable() {
    public void run() {
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESSING_TIME_PER_SECOND_LAST) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESSING_TIME_PER_SECOND_IN_WINDOW) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.VIEW_PROCESSES_COUNT) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESS_CPU_TIME_FRACTION_IN_WINDOW) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESSING_TIME_PER_SECOND_LAST_PER_NODE) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESSING_TIME_PER_SECOND_IN_WINDOW_PER_NODE) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.VIEW_PROCESSES_COUNT_PER_NODE) >= 0);
        assertTrue(cluster.getAttribute(OpenGammaMonitoringAggregation.PROCESS_CPU_TIME_FRACTION_IN_WINDOW_PER_NODE) >= 0);
    }});

Hopefully that should fail with the current code, where the summing sensors are not currently being set.