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.

Beginne mit dem Lesen von Issue #701 und der darin referenzierten Implementierung in multiprocess.py. Das Issue beschreibt Schwierigkeiten bei der Trennung mehrerer Collector-Instanzen, definiert jedoch weder eine Implementierung noch Akzeptanzkriterien. Kläre daher das beabsichtigte Verhalten und den Umfang, bevor du feststellst, was als abgeschlossen gelten würde.