show relay state in prometheus metrics

[kabakaev]

Description:

I use Tasmota as a thermostat to switch heater relay on and off.
Then, a Prometheus instance scrapes the metrics exporter at the tasmota thermostat to collect temperature and relay state gauges.

Current development branch already exports temperature data on /metrics, but it didn't export the relay states.

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works on Tasmota core ESP8266 V.2.7.4.9
• The code change is tested and works on Tasmota core ESP32 V.1.0.4.2
• I accept the CLA.