Tracing with Zipkin doesn't appear to work

[benmoss]

Describe the bug
Tracing support doesn't appear to be working

Expected behavior
I've configured my broker with the sample configs from eventing to have a Zipkin service running in knative-eventing and a config-tracing that forwards with a sample-rate of 1.0.

I configured the dispatcher to log at DEBUG level and see these lines in the logs:

{"@timestamp":"2021-07-15T15:37:07.873Z","@version":"1","message":"Registering tracing configurations backend=ZIPKIN sampleRate=1.0 loggingDebugEnabled=true","logger_name":"dev.knative.eventing.kafka.broker.core.tracing.TracingConfig","thread_name":"main","level":"INFO","level_value":20000,"backend":"ZIPKIN","sampleRate":1.0,"loggingDebugEnabled":true}
{"@timestamp":"2021-07-15T15:37:07.972Z","@version":"1","message":"Add Zipkin simple processor","logger_name":"dev.knative.eventing.kafka.broker.core.tracing.TracingConfig","thread_name":"main","level":"DEBUG","level_value":10000}

So it looks like it's picking up the config, but I don't see any traces being emitted in my Zipkin instance. I have a simple heartbeat source emitting to an event-display service, nothing fancy.

I tried using ksniff to inspect packets coming out of the dispatcher and confirmed there's nothing going out to the zipkin server.

To Reproduce
Configure tracing, see if it works at all?

Knative release version
0.24

Additional context
knative/eventing#5589 reports that it's not working with Jaeger as well

[pierDipi]

I see them:

Just a couple of notes:

• We don't generate the trace context if not present but we propagate it, so you need to make sure that the source is capable of sending trace metadata
• We support only w3c, b3 in WIP Support B3 headers in OTEL propagation  #725
• config-tracing that I used is this CM https://github.com/knative-sandbox/eventing-kafka-broker/blob/main/test/config/100-config-tracing.yaml

[benmoss]

Ah, I think I was under the impression that it would be generated if not present. Thanks for the follow-up!