code: def begin_span(self, name, span_type, context=None, leaf=False, tags=None):
          
          return self._begin_span(name, span_type, context=context, leaf=leaf, tags=tags, parent_span_id=None)
docs: Begin a new span
          :param name: name of the span
          :param span_type: type of the span
          :param context: a context dict
          :param leaf: True if this is a leaf span
          :param tags: a flat string/string dict of tags
          :return: the Span object