Add icon to Plex sensor (#30172)

homeassistant/components/plex/sensor.py

@@ -86,6 +86,11 @@ class PlexSensor(Entity):
         """Return the unit this state is expressed in."""
         return "Watching"
 
+    @property
+    def icon(self):
+        """Return the icon of the sensor."""
+        return "mdi:plex"
+
     @property
     def device_state_attributes(self):
         """Return the state attributes."""