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:

django-prometheus/django_prometheus/exports.py

Lines 114 to 127 in 434a3ba

	def ExportToDjangoView(request):
	"""Exports /metrics as a Django view.
	You can use django_prometheus.urls to map /metrics to this view.
	"""
	if "prometheus_multiproc_dir" in os.environ:
	registry = prometheus_client.CollectorRegistry()
	multiprocess.MultiProcessCollector(registry)
	else:
	registry = prometheus_client.REGISTRY
	metrics_page = prometheus_client.generate_latest(registry)
	return HttpResponse(
	metrics_page, content_type=prometheus_client.CONTENT_TYPE_LATEST
	)