Expose the used registry so custom collectors can be used

[wolph]

When trying to use a custom collector (https://github.com/prometheus/client_python#custom-collectors) the results don't show up when the prometheus_multiproc_dir is set. The problem is that the registry as defined at line 120 here is not accessible to register the collectors: https://github.com/korfuri/django-prometheus/blob/434a3ba36bdada45c9633451f5f6cfd145814ccf/django_prometheus/exports.py#L114-L127