Example for using tracing extension and messages

[liamwh]

Is your feature request related to a problem? Please describe.

I am investigating how we could leverage CloudEvents within my org, but am uncertain how we can include tracing information in a CloudEvent so that traces can be followed across service boundaries and correlated by Jaeger / Tempo.

For example: One service is written in Rust, and it leverages the tracing crate. The functions are leveraging the instrument macro and with tracing configured as seen here. We'd like to publish a CloudEvent externally to the application (Kafa for example), and we see that we should use this extension extension API, but there no examples how to use it.

Describe the solution you'd like

An example added to the /examples folder where an event is published that includes tracing information according to the CloudEvents distributed tracing spec whilst using the tracing crate, the most prominent tracing crate for Rust.

   let example_event = cloudevents::EventBuilderV10::new()
        .id(Ulid::new())
        .source(SOURCE_NAME)
        .ty(EXAMPLE_EVENT_TYPE)
        .time(chrono::Utc::now())
        .subject(example_object.id)
        .data(
            mime::APPLICATION_OCTET_STREAM.to_string(),
            bincode::serialize(&example_object)?,
        )
        .extension("traceparent", bincode::serialize(&WHAT_GOES_HERE)) // <-- How to get values as required by traceparent here?
        .build()?;

[liamwh]

Found the solution. I still think adding an example is a good idea, and I'm also wondering if a builder param behind a feature flag makes sense here to automate all this boilerplate?

 let builder = cloudevents::EventBuilderV10::new()
      .id(Ulid::new())
      .source(SOURCE_NAME)
      .ty(EXAMPLE_EVENT_TYPE)
      .time(chrono::Utc::now())
      .subject(example_object.id)
      .data(
          mime::APPLICATION_OCTET_STREAM.to_string(),
          bincode::serialize(&example_object)?,
      )

  let span = tracing::Span::current();
  let otel_context = span.context();
  let span = otel_context.span();
  let span_context = span.span_context();
  if span_context.is_sampled() {
      let trace_id = span_context.trace_id().to_bytes();
      let span_id = span_context.span_id().to_bytes();
      let trace_flags = span_context.trace_flags().to_u8();
      let trace_id_hex = hex::encode(trace_id);
      let span_id_hex = hex::encode(span_id);
      let traceparent = format!("00-{}-{}-{:02x}", trace_id_hex, span_id_hex, trace_flags);
      builder = builder.extension("traceparent", traceparent);
  }
  let example_event = builder.build()?;

[Lazzaretti]

That would be nice (example & feature flag)! I will happily review a PR if you have time to contribute something.