No eventing-level spans in tracing visualization

### Describe the bug
We're using the tracing setup described in the [knative docs](https://knative.dev/blog/articles/distributed-tracing/) where we can see the request progressing across knative components in Jaeger UI. After having to reinstall the tracing setup (for infrastructure-related reasons), the eventing components don't show up in the traces anymore.

### Expected behavior
Request traces include eventing-level traces, for example like this (an old screenshot):

### Actual behavior
The trace only includes serving-level traces:

**To Reproduce**
- Setup tracing as described in the docs
- Deploy a knative service and a knative kafka source that simply redirects requests to the service
- Send a request and search for the trace in Jaeger UI

**Knative release version**
- Knative serving: v1.12.0
- Knative eventing core: v1.12.6
- Knative kafka source: v1.12.9

Start by reproducing the tracing setup from the linked Knative docs with a Knative service and Kafka source, then inspect the resulting trace in Jaeger UI. Compare the serving-level spans with the expected eventing-level spans for the listed Knative v1.12 releases. Done means eventing components appear in request traces again.

Written by the indexing model from the issue text.