code: func (t *Tracer) InjectHTTPHeaders(r *http.Request) {
  	if span := opentracing.SpanFromContext(r.Context()); span != nil {
  		if err := t.tracer.Inject(
  			span.Context(),
  			opentracing.HTTPHeaders,
  			opentracing.HTTPHeadersCarrier(r.Header),
  		); err != nil {
  			log.Printf("opentracing inject error: %s", err)
  		}
  	}
  }
docs: // InjectHTTPHeaders adds the required HTTP headers to pass context between nodes.