code: def span(self, name='child_span'):
          
          child_span = BlankSpan(name, parent_span=self)
          self._child_spans.append(child_span)
          return child_span
docs: Create a child span for the current span and append it to the child
          spans list.
  
          :type name: str
          :param name: (Optional) The name of the child span.
  
          :rtype: :class: `~opencensus.trace.blankspan.BlankSpan`
          :returns: A child Span to be added to the current span.