Fix Requests.is_ajax() deprecation warning in Django 3.1 (#242)

See the Django 3.1 release notes for information about deprecating the
`Requests.is_ajax()` method.

https://docs.djangoproject.com/en/3.1/releases/3.1/

Author: lithammer

django_prometheus/middleware.py

@@ -246,8 +246,11 @@ def process_request(self, request):
         self.label_metric(
             self.metrics.requests_by_transport, request, transport=transport
         ).inc()
-        if request.is_ajax():
+
+        # Mimic the behaviour of the deprecated "Request.is_ajax()" method.
+        if request.META.get("HTTP_X_REQUESTED_WITH") == "XMLHttpRequest":
             self.label_metric(self.metrics.requests_ajax, request).inc()
+
         content_length = int(request.META.get("CONTENT_LENGTH") or 0)
         self.label_metric(self.metrics.requests_body_bytes, request).observe(
             content_length