code: func (t *Tracer) Extract(extractor propagation.Extractor) (sc model.SpanContext) {
  	if atomic.LoadInt32(&t.noop) == 1 {
  		return
  	}
  	psc, err := extractor()
  	if psc != nil {
  		sc = *psc
  	}
  	sc.Err = err
  	return
  }
docs: // Extract extracts a SpanContext using the provided Extractor function.