Fix use of dedicated listeners with multiprocess

[riconnon]

Extract the code to select the registry used for the exporter from the
Django view and provide the registry as a parameter to clients for
deciated listeners as well.

[riconnon]

Hi @korfuri
I've just rebased this over your latest, and I wondered if you might have time to take a look at this.

For me, without this, I'm unable to make a working setup using multiprocess and dedicated listeners.

[asherf]

please provide some more context for this change..
What problem does it solve ? what is the use case for this ?

[riconnon]

@asherf
I'm using a dedicated WSGI listener for metrics collection, and multiprocess mode to support my WSGI/ASGI running multiple processes.
With the code from master this didn't work, and I ended up with no metrics because the listener was not using the same registry as the multiprocess collector.

[tykling]

I think I might be facing this issue. Any update on merging this? @riconnon if you understand the issue well enough maybe you can include a testcase which demonstrates it, although I understand it is not trivial given the nature of multiproc stuff.

[riconnon]

@asherf It would be really useful to see this PR reviewed since without it my setup is entirely broken