code: final MetricDatum cloneMetricDatum(MetricDatum md) {
          return new MetricDatum()
              .withDimensions(md.getDimensions()) // a new collection is created
              .withMetricName(md.getMetricName())
              .withStatisticValues(md.getStatisticValues())
              .withTimestamp(md.getTimestamp())
              .withUnit(md.getUnit())
              .withValue(md.getValue());
      }
docs: Returns a metric datum cloned from the given one.
  Made package private only for testing purposes.