code: func ShiftSpanByKey(k string) opentracing.Span {
  	if !Enabled {
  		return nil
  	}
  	span, spanOk := store.spans.Load(k)
  	if !spanOk {
  		return nil
  	}
  	store.spans.Delete(k)
  	return span.(opentracing.Span)
  }
docs: // ShiftSpanByKey retrieves the span stored under the key of the string given as argument
  // The span is then deleted from the store