Add Django exporter (#1088)

[Chadys]

Fixes #1088

Finally got the time to propose what I had in mind :)

I added a utility view for easy inclusion of a metrics endpoint inside a Django project.

I also added some explanation about django-prometheus and the difference between what it provides and the new view. I hope the difference is clear that way.

[Chadys]

I can't seem to satisfy both isort and flake8_import_order, hence the CI error