v-m: Add alert for sensor battery levels

I did not realize the batteries on the garage door tilt sensors had
died.  Adding alerts for various sensor batteries should help keep me
better informed.

victoria-metrics/alerts.yml

@@ -120,3 +120,9 @@ groups:
   - alert: Frigate is Unavailable
     expr:
       homeassistant_entity_available{entity=~".*frigate_(server|status)"} != 1
+
+- name: Sensors
+  rules:
+  - alert: Battery Low
+    expr:
+      homeassistant_sensor_battery_percent{entity!~"sensor\\.(pixel_|sm_p610).*"} < 10