Add usage example for Celery

I recently wanted to collect custom application metrics from a rather large Celery app. I tried several approaches with varying degrees of awkwardness, until finally settling on having a "sidecar" HTTP server process for all Celery nodes.
This works pretty well, but took me a while to get right. To spare others from wasting as much time as me, I would like to propose adding a section on getting Celery metrics set up with the Prometheus client. I wrote up the solution on StackOverflow: https://stackoverflow.com/a/75799358/2532203

If you're interested, I can properly summarise it and open a PR?

Also, if you look at the code, I needed to copy some swaths of the library because the built-in Prometheus WSGI server is hard-coded to demonise the thread; this causes trouble when running in a demonised process. Maybe this could be made optional with another keyword argument that defaults to True?

Beginne mit der im Issue verlinkten StackOverflow-Lösung und der Referenz zum WSGI-Server des Prometheus-Clients in exposition.py. Fasse die Einrichtung des Sidecar-HTTP-Servers zum Erfassen von Metriken aus Celery-Workern im passenden Dokumentationsabschnitt zusammen und überprüfe, dass das Beispiel vollständig und verständlich ist. Behandle das optionale daemon-thread-Verhalten als separate Frage, sofern die bestehende Diskussion es nicht klärt.