Support unique_id

Support is a bit fragile since we don't really have a serial number to
base on. However we have a reasonnable default (source prometheus +
expression) and a way for user to specify id easily if they to change
the prometheus query.

custom_components/prometheus_query/sensor.py

@@ -22,6 +22,7 @@ from prometheus_client import Summary
 CONF_PROMETHEUS_URL = 'prometheus_url'
 CONF_PROMETHEUS_QUERY = 'prometheus_query'
 CONF_STATE_CLASS = 'state_class'
+CONF_UNIQUE_ID = 'unique_id'
 SCAN_INTERVAL = timedelta(seconds=600)
 
 _LOGGER = logging.getLogger(__name__)
@@ -33,6 +34,7 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Required(CONF_NAME): cv.string,
     vol.Optional(CONF_UNIT_OF_MEASUREMENT): cv.string,
     vol.Optional(CONF_STATE_CLASS): STATE_CLASSES_SCHEMA,
+    vol.Optional(CONF_UNIQUE_ID): cv.string,
 })
 
 def setup_platform(hass, config, add_entities, discovery_info=None):
@@ -43,6 +45,7 @@ def setup_platform(hass, config, add_entities, discovery_info=None):
         'name': str(config.get(CONF_NAME)),
         'unit': str(config.get(CONF_UNIT_OF_MEASUREMENT)),
         'state_class': str(config.get(CONF_STATE_CLASS)),
+        'unique_id': str(config.get(CONF_UNIQUE_ID)),
     }
     add_entities([PrometheusQuery(prom_data)], True)
 
@@ -57,6 +60,9 @@ class PrometheusQuery(Entity):
         self._state = None
         self._unit_of_measurement = prom_data["unit"]
         self._state_class = prom_data["state_class"]
+        self._attr_unique_id = f"${prom_data['url']}$${prom_data['query']}"
+        if prom_data["unique_id"] is not None:
+            self._attr_unique_id = prom_data["unique_id"]
 
     @property
     def name(self):