Migrate Django middleware to new-style

New-style middlewares were [introduced][django_1_10_changelog] in Django
`1.10`, and also `settings.MIDDLEWARE_CLASSES` was
[removed][django_2_0_changelog] in Django 2.0.

This change migrates the Django middleware to conform with the new
style. This is useful because it will help us solve the pending issue in
open-telemetry#391.

By having a single entrypoint to the middleware, `__call__`, which is
[wrapped with `sync_to_async` just once][call_wrapped] for async
requests, we avoid the [issue][asgiref_issue] where a `ContextVar`
cannot be reset from a different context.

With the current deprecated `MiddlewareMixin` way, both `process_request`
and `process_response` were being
[wrapped separately with `sync_to_async`][mixin_wrapped], which was the
source of the mentioned issue.

[django_1_10_changelog]: https://docs.djangoproject.com/en/3.2/releases/1.10/#new-style-middleware
[django_2_0_changelog]: https://docs.djangoproject.com/en/3.2/releases/2.0/#features-removed-in-2-0
[call_wrapped]: https://github.com/django/django/blob/213850b4b9641bdcb714172999725ec9aa9c9e84/django/core/handlers/base.py#L54-L57
[mixin_wrapped]: https://github.com/django/django/blob/213850b4b9641bdcb714172999725ec9aa9c9e84/django/utils/deprecation.py#L137-L147
[asgiref_issue]: django/asgiref#267

Author: adamantike

instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/middleware.py

@@ -42,11 +42,6 @@
 except ImportError:
     from django.urls import Resolver404, resolve
 
-try:
-    from django.utils.deprecation import MiddlewareMixin
-except ImportError:
-    MiddlewareMixin = object
-
 _logger = getLogger(__name__)
 _attributes_by_preference = [
     [
@@ -70,7 +65,7 @@
 ]
 
 
-class _DjangoMiddleware(MiddlewareMixin):
+class _DjangoMiddleware:
     """Django Middleware for OpenTelemetry"""
 
     _environ_activation_key = (
@@ -89,6 +84,9 @@ class _DjangoMiddleware(MiddlewareMixin):
         [Span, HttpRequest, HttpResponse], None
     ] = None
 
+    def __init__(self, get_response):
+        self.get_response = get_response
+
     @staticmethod
     def _get_span_name(request):
         try:
@@ -111,6 +109,11 @@ def _get_span_name(request):
         except Resolver404:
             return "HTTP {}".format(request.method)
 
+    def __call__(self, request):
+        self.process_request(request)
+        response = self.get_response(request)
+        return self.process_response(request, response)
+
     def process_request(self, request):
         # request.META is a dictionary containing all available HTTP headers
         # Read more about request.META here: