code: func InjectTrace(r *http.Request) {
  	if span := opentracing.SpanFromContext(r.Context()); span != nil {
  		span.Tracer().Inject(
  			span.Context(),
  			opentracing.HTTPHeaders,
  			opentracing.HTTPHeadersCarrier(r.Header),
  		)
  	}
  }
docs: // InjectTrace writes any span from the request's context into the request headers.