def sorted_metrics(metrics, hierarchy)
      # 'hierarchy' is a hash where the keys are metric names and the values
      # are arrays with the names of the children metrics. Only metrics with
      # children and with at least one usage limit appear as keys.
      parent_metrics = hierarchy.keys
      child_metrics = metrics - parent_metrics
      parent_metrics + child_metrics
    end
def sorted_metrics(metrics, hierarchy)
      # 'hierarchy' is a hash where the keys are metric names and the values
      # are arrays with the names of the children metrics. Only metrics with
      # children and with at least one usage limit appear as keys.
      parent_metrics = hierarchy.keys
      child_metrics = metrics - parent_metrics
      parent_metrics + child_metrics
    end
Returns an array of metric names. The array is guaranteed to have all the
 parents first, and then the rest.
 In 3scale, metric hierarchies only have 2 levels. In other words, a
 metric that has a parent cannot have children.