Setting http.response.body.size attribute in parent span

Question

I'm using OpenTelemetry Go SDK and I'm trying to set http.response.body.size attribute in my parent span. I'm using otelhttp.NewHandler() to serve the HTTP request:

func NewHTTPHandler(name string, handler http.Handler, tp Provider, attr ...Attribute) http.Handler {
    opts := []otelhttp.Option{
        otelhttp.WithSpanNameFormatter(httpSpanNameFormatter),
    }

    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        attr = append(attr, NewAttribute("http.request.body.size", r.ContentLength))

        opts = append(opts, otelhttp.WithSpanOptions(
            trace.WithAttributes(attr...),
        ))

        otelhttp.NewHandler(handler, name, opts...).ServeHTTP(w, r)
    })
}

At the moment, I could store the http.request.body.size value before processing the HTTP request, but I don't know how to do something similar with the response size.

I tried to wrap the response writer with a callback in order to store the size bytes, and it's working, but when I try to get the span using the request context in order to store this attribute, the span is empty (SpanID is "000000000000"), I could notice it by printing it:

type responseWriterWrapper struct {
    http.ResponseWriter
    contentLength int
    onWrite       func(int) // callback for when Write is called
}

func (rw *responseWriterWrapper) Write(b []byte) (int, error) {
    n, err := rw.ResponseWriter.Write(b)
    rw.contentLength += n
    rw.onWrite(rw.contentLength)
    return n, err
}

func NewHTTPHandler(name string, handler http.Handler, tp Provider, attr ...Attribute) http.Handler {
    opts := []otelhttp.Option{
        otelhttp.WithSpanNameFormatter(httpSpanNameFormatter),
    }

    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        attr = append(attr, NewAttribute("http.request.body.size", r.ContentLength))

        opts = append(opts, otelhttp.WithSpanOptions(
            trace.WithAttributes(attr...),
        ))

        // Setup callback for when Write is called
        rwWrapper := &responseWriterWrapper{
            ResponseWriter: w,
            onWrite: func(length int) {
                span := trace.SpanFromContext(r.Context())
                fmt.Println("SPAN ID:", span.SpanContext().SpanID())
                if span != nil {
                    span.SetAttributes(NewAttribute("http.response.body.size", int64(length)))
                }
            },
        }

        otelhttp.NewHandler(handler, name, opts...).ServeHTTP(rwWrapper, r)
    })
}

Setting http.response.body.size attribute in parent span

0 Answers0