code: def _check_span_id(self, span_id):
          
          if span_id is None:
              return None
          assert isinstance(span_id, six.string_types)
  
          if span_id is INVALID_SPAN_ID:
              logging.warning(
                  'Span_id {} is invalid (cannot be all zero)'.format(span_id))
              self.from_header = False
              return None
  
          match = SPAN_ID_PATTERN.match(span_id)
  
          if match:
              return span_id
          else:
              logging.warning(
                  'Span_id {} does not the match the '
                  'required format'.format(span_id))
              self.from_header = False
              return None
docs: Check the format of the span_id to ensure it is 16-character hex
          value representing a 64-bit number. If span_id is invalid, logs a
          warning message and returns None
  
          :type span_id: str
          :param span_id: Identifier for the span, unique within a span.
  
          :rtype: str
          :returns: Span_id for the current span.