code: def span_context_to_string(trace_id, span_id, parent_id, flags):
      
      parent_id = parent_id or 0
      return '{:x}:{:x}:{:x}:{:x}'.format(trace_id, span_id, parent_id, flags)
docs: Serialize span ID to a string
          {trace_id}:{span_id}:{parent_id}:{flags}
  
      Numbers are encoded as variable-length lower-case hex strings.
      If parent_id is None, it is written as 0.
  
      :param trace_id:
      :param span_id:
      :param parent_id:
      :param flags: