code: def start_child_span(operation_name, tracer=None, parent=None, tags=None):
      
      tracer = tracer or opentracing.tracer
      return tracer.start_span(
          operation_name=operation_name,
          child_of=parent.context if parent else None,
          tags=tags
      )
docs: Start a new span as a child of parent_span. If parent_span is None,
      start a new root span.
  
      :param operation_name: operation name
      :param tracer: Tracer or None (defaults to opentracing.tracer)
      :param parent: parent Span or None
      :param tags: optional tags
      :return: new span