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

[csmarchbanks]

One comment, but generally looks reasonable to me, thanks and apologies for the slow review!

[Chadys]

No worries, volunteer's work knows no clock ;)
Thanks for the review, should be good now

[csmarchbanks]

Thanks!