code: func (d *tracedDoer) Do(ctx context.Context, req *http.Request) (*http.Response, error) {
  	var (
  		traceID = ContextTraceID(ctx)
  		spanID  = ContextSpanID(ctx)
  	)
  	if traceID != "" {
  		req.Header.Set(TraceIDHeader, traceID)
  		req.Header.Set(ParentSpanIDHeader, spanID)
  	}
  
  	return d.Doer.Do(ctx, req)
  }
docs: // Do adds the tracing headers to the requests before making it.