Support multiprocess mode without setting an environment variable

This is essentially a duplicate of issue #701

The main problem for my use case is that it's impossible to separate multiple instances of collectors, even though it has a path field misleading you into thinking that you can
https://github.com/prometheus/client_python/blob/master/prometheus_client/multiprocess.py#L22

I am personally not sure about ways of fixing that, apart from just unifying the implementations and only supporting the multiprocess mode. Because running python as multiple processes is arguably a more popular way of doing things, Web servers, and async processing all pretty much run on multiprocessing and it has to be the largest chunk of software that wants to be instrumented.

And while mmapped files have overhead, it's not super big and python is not used for blazingly fast things in general, so IMO it's a reasonable tradeoff for the simplicity of maintaining a single mode of operation.

先阅读 issue #701 以及其中引用的 multiprocess.py 实现。该 issue 描述了分离多个 Collector 实例的困难，但没有定义实现方案或验收标准，因此在确定什么才算完成之前，先明确预期行为和范围。