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?

Start with the StackOverflow solution linked in the issue and the Prometheus client's exposition.py WSGI server reference. Summarize the sidecar HTTP-server setup for collecting metrics from Celery workers in the appropriate documentation section, and verify that the example is complete and understandable. Treat the optional daemon-thread behavior as a separate question unless the existing discussion resolves it.