Add target high/low temperatures to prometheus integration (#50071)

* add target high/low temperatures to prometheus integration

* use labels

* Revert "use labels"

This reverts commit 09c56d6359.

* fix naming

* tests

* cleanup

* use three separate metrics

* fix descriptions

tests/components/prometheus/test_init.py

@@ -137,6 +137,24 @@ async def test_view_empty_namespace(hass, hass_client):
         'friendly_name="HeatPump"} 25.0' in body
     )
 
+    assert (
+        'climate_target_temperature_celsius{domain="climate",'
+        'entity="climate.heatpump",'
+        'friendly_name="HeatPump"} 20.0' in body
+    )
+
+    assert (
+        'climate_target_temperature_low_celsius{domain="climate",'
+        'entity="climate.ecobee",'
+        'friendly_name="Ecobee"} 21.0' in body
+    )
+
+    assert (
+        'climate_target_temperature_high_celsius{domain="climate",'
+        'entity="climate.ecobee",'
+        'friendly_name="Ecobee"} 24.0' in body
+    )
+
     assert (
         'humidifier_target_humidity_percent{domain="humidifier",'
         'entity="humidifier.humidifier",'