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?

Commencez par la solution StackOverflow liée dans l’issue et par la référence du serveur WSGI du client Prometheus dans exposition.py. Résumez la configuration du serveur HTTP sidecar pour collecter les métriques des workers Celery dans la section de documentation appropriée, et vérifiez que l’exemple est complet et compréhensible. Traitez le comportement optionnel de daemon-thread comme une question distincte, sauf si la discussion existante la résout.