Add unit of measurement to Tautulli sensor (#23873)

Adds unit of measurement ("Watching") to sensor, so it can be graphed properly.
This is the same unit of measurement as the Plex sensor.

homeassistant/components/tautulli/sensor.py

@@ -119,6 +119,11 @@ class TautulliSensor(Entity):
         """Return the icon of the sensor."""
         return 'mdi:plex'
 
+    @property
+    def unit_of_measurement(self):
+        """Return the unit this state is expressed in."""
+        return "Watching"
+
     @property
     def device_state_attributes(self):
         """Return attributes for the sensor."""