metrics: Make metrics actually usable

With this commit, we should get stats for the item count per event per
returned-state.

core/core/management/commands/update_metrics.py

@@ -1,8 +0,0 @@
-from django.core.management.base import BaseCommand, CommandError
-from core.metrics import update_item_count
-
-class Command(BaseCommand):
-    help = "Refresh the metrics"
-    
-    def handle(self, *args, **options):
-        update_item_count()

core/core/metrics.py

@@ -1,11 +1,35 @@
-from prometheus_client import Gauge
-from django_prometheus import exports
-from inventory.models import *
+from django.apps import apps
+from prometheus_client.core import CounterMetricFamily, REGISTRY
+from django.db.models import Case, Value, When, BooleanField, Count
+from inventory.models import Item
 
-g_items_total = Gauge('c3lf_items_total', 'Current Total items')
+class ItemCountCollector(object):
 
-def update_item_count():
-    # Get the count of MyModel objects
-    count = Item.objects.count()
-    # Set the gauge to the current count
-    g_items_total.set(count)
+    def collect(self):
+        counter = CounterMetricFamily("item_count", "Current number of items", labels=['event', 'state'])
+
+        yield counter
+
+        if not apps.models_ready or not apps.apps_ready:
+            return
+
+        queryset = (
+            Item.all_objects
+                .annotate(
+                    returned=Case(
+                        When(returned_at__isnull=False, then=Value(False)),
+                        default=Value(True),
+                        output_field=BooleanField()
+                    )
+                )
+                .values('event__slug', 'returned', 'event_id')
+                .annotate(amount=Count('id'))
+                .order_by('event__slug', 'returned')  # Optional: order by slug and returned
+        )
+
+        for e in queryset:
+            counter.add_metric([e["event__slug"].lower(), str(e["returned"])], e["amount"])
+
+        yield counter
+
+REGISTRY.register(ItemCountCollector())

core/core/settings.py

@@ -70,7 +70,6 @@ INSTALLED_APPS = [
     'inventory',
     'mail',
     'notify_sessions',
-    'core'
 ]
 
 REST_FRAMEWORK = {