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?

从 issue 中链接的 StackOverflow 解决方案以及 Prometheus 客户端在 exposition.py 中的 WSGI 服务器参考开始。将用于从 Celery workers 收集指标的 sidecar HTTP-server 设置总结到适当的文档部分，并确认示例完整且易于理解。除非现有讨论已经解决，否则将可选的 daemon-thread 行为作为单独的问题处理。