Add metric 'shift_registrations'

shiftregister/metrics/views.py

@@ -73,6 +73,15 @@ def metrics(request):
                         "messages_sent_total",
                         Message.objects.all().count(),
                     ),
+                    (
+                        "shift_registrations",
+                        ShiftRegistration.objects.filter(
+                            state__in=(
+                                ShiftRegistration.RegState.REGISTERED,
+                                ShiftRegistration.RegState.CHECKED_IN,
+                            )
+                        ).count(),
+                    ),
                     (
                         "worked_seconds_total",
                         worked_seconds_total.total_seconds()