Change to use the waiting tasks in the filter for task metrics.

Closes #5941

After some discussion we decided to use the task status in the filter
that search for unblocked tasks in the queue. This can avoid a situation
where tasks have the `unblocked_at` field filled yet they are not
waiting in the queue anymore.

(cherry picked from commit b78977c)

CHANGES/5941.bugfix

@@ -0,0 +1 @@
+Fixed incorrect count of waiting tasks in the queue for the `tasks_unblocked_queue` and `tasks_longest_unblocked_time` metrics.

pulpcore/tasking/worker.py

@@ -483,7 +483,7 @@ def _record_unblocked_waiting_tasks_metric(self):
             ):
                 # For performance reasons we aggregate these statistics on a single database call.
                 unblocked_tasks_stats = (
-                    Task.objects.filter(unblocked_at__isnull=False, started_at__isnull=True)
+                    Task.objects.filter(unblocked_at__isnull=False, state=TASK_STATES.WAITING)
                     .annotate(unblocked_for=Value(timezone.now()) - F("unblocked_at"))
                     .aggregate(
                         longest_unblocked_waiting_time=Max(