Assign HttpRequest.resolver_match for dd-trace (#5)

Once a request is determined to be a health or readiness check, its
[`resolver_match`][0] attribute is assigned so that [ddtrace][1] can
give its span a proper resource name ("GET /healthz" or "GET /readiness").

Referenced ddtrace implementation: [`/ddtrace/contrib/django/utils.py`][2]

[0]: https://docs.djangoproject.com/en/3.2/ref/request-response/#django.http.HttpRequest.resolver_match
[1]: https://pypi.org/project/ddtrace/
[2]: https://github.com/DataDog/dd-trace-py/blob/bf9587d2f4afa1485e9dd5232b23f91b941b206f/ddtrace/contrib/django/utils.py#L182-L201

django_middleware/healthchecks.py

@@ -118,10 +118,15 @@ def __init__(self, get_response):
         self.get_response = get_response
 
     def __call__(self, request) -> HttpResponse:
+        from django.urls import ResolverMatch
+
         if request.method != "GET" or request.path not in self.endpoints:
             # Passthrough if we aren't accessing health checks.
             return self.get_response(request)
 
+        # Add a resolver_match so that dd-trace can give the span a proper resource.
+        request.resolver_match = ResolverMatch(self, (request,), {}, route=request.path)
+
         if request.path == READINESS_ENDPOINT:
             # Throw an exception if checks don't pass.
             return check_readiness()