Add a mandatory intro to tracing to the end of kafka-cron

[illicitonion]

kafka-cron introduces prometheus metrics in a really useful way.

The following project, raft, has a lot of new concepts and design space for the fellows.

There are some simple, meaningful tracing points we could add to kafka cron to introduce them to the basic concepts of tracing, so there are fewer new concepts for them to learn all at once in raft; I think we should consider doing so.

This could fall into sprint 5 (as an "intro to tracing"), rather than sprint 4 (as a "rest of kafka-cron"), if we wanted.