Add an obvious notice for metric initialization of multiprocess collector

[justdoit0823]

As a beginner, I build a histogram metric in my Python application with a self-defined collect registry.When deploy under multiple process circumstance, I get a help from README file. According to that, I don't change metric's initialzation and get http 500 when making requests to pushgateway. Finally, I fix the problem and think there should be a more clear and obvious description for beginners.

[brian-brazil]

The example already indicates how to do it correctly. I don't think it's sane to list every way in why someone managed to not follow the example correctly.

[justdoit0823]

while there is no any more explaination about CollectorRegistry class, collector will fail when using multiprocess collector's registry as metric registry argument. It just like a balck box.

[brian-brazil]

This is all deep magic. Attempting to explain it would only cause more user confusion.

[justdoit0823]

With more detailed document about how CollectorRegitry works with metric, beginners can easily make sense of the structure of metric observation and metric collector.

[brian-brazil]

Users shouldn't have to care. The docs provide full examples on how to use this correctly, and anyone looking for more can read the code or ask for help.