Add prometheus metric naming guidelines (#37149)

* Add prometheus metric naming guidelines.

* Update homeassistant/components/prometheus/README.md

Co-authored-by: Martin Weinelt <mweinelt@users.noreply.github.com>

Co-authored-by: Martin Weinelt <mweinelt@users.noreply.github.com>

homeassistant/components/prometheus/README.md

+# Prometheus integration
+
+This integration exposes metrics in a Prometheus compatible format.
+
+## Metric naming guidelines
+
+Please follow these guidelines while defining metrics.
+
+* Metric and label names should conform to [Prometheus
+  naming guidelines](https://prometheus.io/docs/practices/naming/).
+* Domain-specific metrics should have the domain (`sensor`, `switch`,
+  `climate`, etc.) as a metric name prefix.
+* Enum-like values (e.g. entity state or current mode) should be exported using
+  a "boolean" metric (values of 0 or 1) broken down by state/mode (as a metric
+  label).