code: func InjectRequestHeaders(r *http.Request) {
  	if span := GetSpan(r); span != nil {
  		err := opentracing.GlobalTracer().Inject(
  			span.Context(),
  			opentracing.HTTPHeaders,
  			HTTPHeadersCarrier(r.Header))
  		if err != nil {
  			log.FromContext(r.Context()).Error(err)
  		}
  	}
  }
docs: // InjectRequestHeaders used to inject OpenTracing headers into the request.