Add unique_id property

homeassistant/components/sensor/systemmonitor.py

@@ -92,6 +92,11 @@ class SystemMonitorSensor(Entity):
         self._state = None
         self._unit_of_measurement = SENSOR_TYPES[sensor_type][1]
 
+    @property
+    def unique_id(self):
+        """Return a unique identifier for the sensor."""
+        return self.type
+
     @property
     def name(self):
         """Return the name of the sensor."""