code: func InjectHTTP(r *http.Request) propagation.Injector {
  	return func(sc model.SpanContext) error {
  		if (model.SpanContext{}) == sc {
  			return ErrEmptyContext
  		}
  
  		if sc.Debug {
  			r.Header.Set(Flags, "1")
  		} else if sc.Sampled != nil {
  			// Debug is encoded as X-B3-Flags: 1. Since Debug implies Sampled,
  			// so don't also send "X-B3-Sampled: 1".
  			if *sc.Sampled {
  				r.Header.Set(Sampled, "1")
  			} else {
  				r.Header.Set(Sampled, "0")
  			}
  		}
  
  		if !sc.TraceID.Empty() && sc.ID > 0 {
  			r.Header.Set(TraceID, sc.TraceID.String())
  			r.Header.Set(SpanID, sc.ID.String())
  			if sc.ParentID != nil {
  				r.Header.Set(ParentSpanID, sc.ParentID.String())
  			}
  		}
  
  		return nil
  	}
  }
docs: // InjectHTTP will inject a span.Context into a HTTP Request