Follow Prometheus metic naming & labeling conventions about django-prometheus

[korfuri]

Thanks for the suggestion! Actually, these metrics get incremented at different points or under different circumstances in Django's handling of the request. For instance a request that doesn't get mapped to a view wouldn't appear in requests_by_view_transport_method but would in the other 3. So I think there's a semantic difference between the seemingly identical metrics.

Feel free to reopen this if there are metrics that I missed and that don't have this particularity, but I think they're all different.