Add generic unavailable and last_updated metrics for prometheus (#37456)

* Add generic unavailable and last_updated metrics for prometheus

* Updated with feedback from the code review

homeassistant/components/prometheus/__init__.py

@@ -25,6 +25,7 @@ from homeassistant.const import (
     CONTENT_TYPE_TEXT_PLAIN,
     EVENT_STATE_CHANGED,
     STATE_ON,
+    STATE_UNAVAILABLE,
     TEMP_CELSIUS,
     TEMP_FAHRENHEIT,
     UNIT_PERCENTAGE,
@@ -153,13 +154,28 @@ class PrometheusMetrics:
 
         handler = f"_handle_{domain}"
 
-        if hasattr(self, handler):
+        if hasattr(self, handler) and state.state != STATE_UNAVAILABLE:
             getattr(self, handler)(state)
 
-        metric = self._metric(
+        labels = self._labels(state)
+        state_change = self._metric(
             "state_change", self.prometheus_cli.Counter, "The number of state changes"
         )
-        metric.labels(**self._labels(state)).inc()
+        state_change.labels(**labels).inc()
+
+        entity_available = self._metric(
+            "entity_available",
+            self.prometheus_cli.Gauge,
+            "Entity is available (not in the unavailable state)",
+        )
+        entity_available.labels(**labels).set(float(state.state != STATE_UNAVAILABLE))
+
+        last_updated_time_seconds = self._metric(
+            "last_updated_time_seconds",
+            self.prometheus_cli.Gauge,
+            "The last_updated timestamp",
+        )
+        last_updated_time_seconds.labels(**labels).set(state.last_updated.timestamp())
 
     def _handle_attributes(self, state):
         for key, value in state.attributes.items():

tests/components/prometheus/test_init.py

@@ -1,9 +1,9 @@
 """The tests for the Prometheus exporter."""
 from dataclasses import dataclass
+import datetime
 
 import pytest
 
-from homeassistant import setup
 from homeassistant.components import climate, humidifier, sensor
 from homeassistant.components.demo.sensor import DemoSensor
 import homeassistant.components.prometheus as prometheus
@@ -16,6 +16,7 @@ from homeassistant.const import (
 )
 from homeassistant.core import split_entity_id
 from homeassistant.setup import async_setup_component
+from homeassistant.util import dt as dt_util
 
 import tests.async_mock as mock
 
@@ -30,21 +31,18 @@ class FilterTest:
     should_pass: bool
 
 
-@pytest.fixture
-async def prometheus_client(loop, hass, hass_client):
+async def prometheus_client(hass, hass_client):
     """Initialize an hass_client with Prometheus component."""
     await async_setup_component(hass, prometheus.DOMAIN, {prometheus.DOMAIN: {}})
 
-    await setup.async_setup_component(
-        hass, sensor.DOMAIN, {"sensor": [{"platform": "demo"}]}
-    )
+    await async_setup_component(hass, sensor.DOMAIN, {"sensor": [{"platform": "demo"}]})
 
-    await setup.async_setup_component(
+    await async_setup_component(
         hass, climate.DOMAIN, {"climate": [{"platform": "demo"}]}
     )
     await hass.async_block_till_done()
 
-    await setup.async_setup_component(
+    await async_setup_component(
         hass, humidifier.DOMAIN, {"humidifier": [{"platform": "demo"}]}
     )
 
@@ -60,7 +58,11 @@ async def prometheus_client(loop, hass, hass_client):
     )
     sensor2.hass = hass
     sensor2.entity_id = "sensor.radio_energy"
-    await sensor2.async_update_ha_state()
+    with mock.patch(
+        "homeassistant.util.dt.utcnow",
+        return_value=datetime.datetime(1970, 1, 2, tzinfo=dt_util.UTC),
+    ):
+        await sensor2.async_update_ha_state()
 
     sensor3 = DemoSensor(
         None, "Electricity price", 0.123, None, f"SEK/{ENERGY_KILO_WATT_HOUR}", None
@@ -89,9 +91,10 @@ async def prometheus_client(loop, hass, hass_client):
     return await hass_client()
 
 
-async def test_view(prometheus_client):  # pylint: disable=redefined-outer-name
+async def test_view(hass, hass_client):
     """Test prometheus metrics view."""
-    resp = await prometheus_client.get(prometheus.API_ENDPOINT)
+    client = await prometheus_client(hass, hass_client)
+    resp = await client.get(prometheus.API_ENDPOINT)
 
     assert resp.status == 200
     assert resp.headers["content-type"] == "text/plain"
@@ -167,6 +170,18 @@ async def test_view(prometheus_client):  # pylint: disable=redefined-outer-name
         'friendly_name="Radio Energy"} 14.0' in body
     )
 
+    assert (
+        'entity_available{domain="sensor",'
+        'entity="sensor.radio_energy",'
+        'friendly_name="Radio Energy"} 1.0' in body
+    )
+
+    assert (
+        'last_updated_time_seconds{domain="sensor",'
+        'entity="sensor.radio_energy",'
+        'friendly_name="Radio Energy"} 86400.0' in body
+    )
+
     assert (
         'sensor_unit_sek_per_kwh{domain="sensor",'
         'entity="sensor.electricity_price",'