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.

Start by reading issue #701 and the referenced multiprocess.py implementation. The issue describes difficulty separating multiple collector instances but does not define an implementation or acceptance criteria, so clarify the intended behavior and scope before identifying what would count as done.