Encode prometheus metric names per the prom spec (#26639)

Referencing issue #26418.

Prometheus metric names can only contain chars a-zA-Z0-9, : and _
(https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels).

HA currently generates invalid prometheus names, e.g. if the unit for a
sensor is a non-ASCII character containing  ° or μ. To resolve, we need
to sanitize the name before creating, replacing non-valid characters
with a valid representation. In this case, I've used
"u{unicode-hex-code}".

Also updated the test case to make sure that the ° case is handled.

homeassistant/components/prometheus/__init__.py

@@ -1,5 +1,6 @@
 """Support for Prometheus metrics export."""
 import logging
+import string
 
 from aiohttp import web
 import voluptuous as vol
@@ -159,10 +160,23 @@ class PrometheusMetrics:
         try:
             return self._metrics[metric]
         except KeyError:
-            full_metric_name = f"{self.metrics_prefix}{metric}"
+            full_metric_name = self._sanitize_metric_name(
+                f"{self.metrics_prefix}{metric}"
+            )
             self._metrics[metric] = factory(full_metric_name, documentation, labels)
             return self._metrics[metric]
 
+    @staticmethod
+    def _sanitize_metric_name(metric: str) -> str:
+        return "".join(
+            [
+                c
+                if c in string.ascii_letters or c.isdigit() or c == "_" or c == ":"
+                else f"u{hex(ord(c))}"
+                for c in metric
+            ]
+        )
+
     @staticmethod
     def state_as_number(state):
         """Return a state casted to a float."""

tests/components/prometheus/test_init.py

@@ -41,6 +41,11 @@ async def prometheus_client(loop, hass, hass_client):
     sensor3.entity_id = "sensor.electricity_price"
     await sensor3.async_update_ha_state()
 
+    sensor4 = DemoSensor("Wind Direction", 25, None, "°", None)
+    sensor4.hass = hass
+    sensor4.entity_id = "sensor.wind_direction"
+    await sensor4.async_update_ha_state()
+
     return await hass_client()
 
 
@@ -103,3 +108,9 @@ def test_view(prometheus_client):  # pylint: disable=redefined-outer-name
         'entity="sensor.electricity_price",'
         'friendly_name="Electricity price"} 0.123' in body
     )
+
+    assert (
+        'sensor_unit_u0xb0{domain="sensor",'
+        'entity="sensor.wind_direction",'
+        'friendly_name="Wind Direction"} 25.0' in body
+    )