step6.go

func serviceHandler(w http.ResponseWriter, r *http.Request) {
    var sp opentracing.Span
    opName := r.URL.Path
    // Attempt to join a trace by getting trace context from the headers.
    wireContext, err := opentracing.GlobalTracer().Extract(
        opentracing.TextMap,
        opentracing.HTTPHeaderTextMapCarrier(r.Header))
    if err != nil {
        // If for whatever reason we can't join, go ahead an start a new root span.
        sp = opentracing.StartSpan(opName)
    } else {
        sp = opentracing.StartSpan(opName, opentracing.ChildOf(wireContext))
    }
  defer sp.Finish()
  // ... rest of the function