code: func (h *RequestID) ServeHTTPContext(ctx context.Context, w http.ResponseWriter, r *http.Request) error {
  	e := h.Extractor
  	if e == nil {
  		e = DefaultRequestIDExtractor
  	}
  	requestID := e(r)
  
  	ctx = httpx.WithRequestID(ctx, requestID)
  	r = r.WithContext(ctx)
  
  	return h.handler.ServeHTTPContext(ctx, w, r)
  }
docs: // ServeHTTPContext implements the httpx.Handler interface. It extracts a
  // request id from the headers and inserts it into the context.