code: protected final StatValue getMonitoredStatValue(T metric) {
          StatValue statValue = getStatValue(metric);
          if (statValue == null)
              throw new IllegalArgumentException("Metric '"+metric.getCaption()+"' is not monitored!");
          return statValue;
      }
docs: Retrieve single StatValue from the internal cache. Method checks if statValue is present and
  throws {@link IllegalArgumentException} if it's not there.
  @param metric Metric from which StatValue was created.
  @return StatValue from the internal cache.
  @throws IllegalArgumentException if internal cache does not contain statValue for the given metric.