Fix prometheus component to fully sanitize Unicode characters (#31037)

* Fix prometheus component to fully santize Unicode characters.

Sanitization used isdigit() to match numerics but that also matches 
Unicode "digit" characters that require special handling, like 
superscripts. Failures would look like this:

ValueError: Invalid metric name: sensor_unit_u0x23_per_m³

Changed sanitize to use string.digits, which matches only ascii digits, 
and added test.

See 
https://stackoverflow.com/questions/44891070/whats-the-difference-between-str-isdigit-isnumeric-and-isdecimal-in-python 
for discussion.

https://docs.python.org/3/library/stdtypes.html#str.isdigit

https://docs.python.org/3.7/library/string.html#string.digits

* fix formatting to comply with black

homeassistant/components/prometheus/__init__.py

@@ -168,7 +168,10 @@ class PrometheusMetrics:
         return "".join(
             [
                 c
-                if c in string.ascii_letters or c.isdigit() or c == "_" or c == ":"
+                if c in string.ascii_letters
+                or c in string.digits
+                or c == "_"
+                or c == ":"
                 else f"u{hex(ord(c))}"
                 for c in metric
             ]

tests/components/prometheus/test_init.py

@@ -46,6 +46,13 @@ async def prometheus_client(loop, hass, hass_client):
     sensor4.entity_id = "sensor.wind_direction"
     await sensor4.async_update_ha_state()
 
+    sensor5 = DemoSensor(
+        None, "SPS30 PM <1µm Weight concentration", 3.7069, None, "µg/m³", None
+    )
+    sensor5.hass = hass
+    sensor5.entity_id = "sensor.sps30_pm_1um_weight_concentration"
+    await sensor5.async_update_ha_state()
+
     return await hass_client()
 
 
@@ -113,3 +120,9 @@ async def test_view(prometheus_client):  # pylint: disable=redefined-outer-name
         'entity="sensor.wind_direction",'
         'friendly_name="Wind Direction"} 25.0' in body
     )
+
+    assert (
+        'sensor_unit_u0xb5g_per_mu0xb3{domain="sensor",'
+        'entity="sensor.sps30_pm_1um_weight_concentration",'
+        'friendly_name="SPS30 PM <1µm Weight concentration"} 3.7069' in body
+    )