code: @Override
      public String getValueByNameAsString(String valueName, String intervalName, TimeUnit timeUnit){
          if (valueName == null || valueName.isEmpty())
              throw new IllegalArgumentException("Value name can not be empty");
          StatValue statValue = statValueMap.get(valueName);
          if (statValue != null) {
              for (T metric : metrics) {
                  if (valueName.equals(metric.getCaption()))
                      return getStatValueAsString(metric, intervalName);
              }
          }
          return super.getValueByNameAsString(valueName, intervalName, timeUnit);
      }
docs: {@inheritDoc}.
  Called by Accumulator.