code: func FlushOnFinish(b bool) SpanOption {
  	return func(t *Tracer, s *spanImpl) {
  		s.flushOnFinish = b
  	}
  }
docs: // FlushOnFinish when set to false will disable span.Finish() to send the Span
  // to the Reporter automatically (which is the default behavior). If set to
  // false, having the Span be reported becomes the responsibility of the user.
  // This is available if late tag data is expected to be only available after the
  // required finish time of the Span.