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.
770eeaf82f