Ignore STATE_UNKNOWN in prometheus (#47840)
Ignore STATE_UNKNOWN in prometheus (#47840)

Giving a value of 0 by default can lead to erroneous
data being exported. For example, if a MQTT temperature
sensor is in `STATE_UNKNOWN` (which can happen after a
HASS restart), a temperature of 0°C will be exported.
Some user might prefer no value rather than a wrong one.
GenericStudent/home-assistant,jawilson/home-assistant,w1ll1am23/home-assistant,nkgilley/home-assistant,rohitranjan1991/home-assistant,home-assistant/home-assistant,lukas-hetzenecker/home-assistant,sander76/home-assistant,kennedyshead/home-assistant,rohitranjan1991/home-assistant,home-assistant/home-assistant,Danielhiversen/home-assistant,sander76/home-assistant,w1ll1am23/home-assistant,toddeye/home-assistant,jawilson/home-assistant,aronsky/home-assistant,mezz64/home-assistant,adrienbrault/home-assistant,toddeye/home-assistant,rohitranjan1991/home-assistant,aronsky/home-assistant,FreekingDean/home-assistant,mezz64/home-assistant,nkgilley/home-assistant,Danielhiversen/home-assistant,lukas-hetzenecker/home-assistant,FreekingDean/home-assistant,adrienbrault/home-assistant,GenericStudent/home-assistant,kennedyshead/home-assistant