code: public Metrics extendMultipleDimensionsIncludeRoot(final String... dimensionsGroup) {
          final String[] dimensionsGroupWithRoot = Arrays.copyOf(dimensionsGroup, dimensionsGroup.length + 1);
          // root dimensions is equivalent to extend root dimensions with no new dimension
          dimensionsGroupWithRoot[dimensionsGroup.length] = getRootDimensionsString();
          return new MetricsGroup(this, dimensionsGroupWithRoot);
      }
docs: Extend the root dimensions with a group of new dimensions, but also include the root dimensions into the group.
  For example:
  The root dimensions is "service=AwesomeService".
  extendMultipleDimensionsIncludeRoot("api=Upload") will create a metrics object which contains
  the root dimensions "service=AwesomeService" and the new one "service=AwesomeService, api=Upload".
  
  @param dimensionsGroup, group of dimensions
  @return metrics object which contains a group of dimensions