Add support for Prometheus/OpenMetrics to One V9

[dechamps]

This commit adds a new feature to the One V9 (ONE_I-9PSL) firmware: support for exposing metrics to Prometheus (or any other ingestor compatible with the OpenMetrics format).

With this change, the AirGradient device will make metrics available on the standard HTTP /metrics endpoint, out-of-the-box, with no need to do anything else. All the user has to do is add their device address as a target to their scrape config on their Prometheus server.

For more information on Prometheus and OpenMetrics, see:

• https://prometheus.io/docs/instrumenting/exposition_formats/
• https://openmetrics.io/
• https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md

This obsoletes projects such as:

• https://github.com/geerlingguy/airgradient-prometheus/tree/ebfa8d0ac6d8c47649e4642b27e90b98557fb8f7/AirGradient-DIY (pinging @geerlingguy)
• https://forum.airgradient.com/t/prometheus-integration/1504

It may be a good idea to mention this on the integrations page after this is deployed.

[dechamps]

For reference, here's an example /metrics response from my device with 760f827:

# HELP airgradient_info AirGradient device information
# TYPE airgradient_info info
airgradient_info{airgradient_serial_number="4XXXXXXXXXXc",airgradient_device_type="ONE_I-9PSL",airgradient_library_version="3.0.4"} 1
# HELP airgradient_config_ok 1 if the AirGradient device was able to successfully fetch its configuration from the server
# TYPE airgradient_config_ok gauge
airgradient_config_ok{} 1
# HELP airgradient_post_ok 1 if the AirGradient device was able to successfully send to the server
# TYPE airgradient_post_ok gauge
airgradient_post_ok{} 1
# HELP airgradient_wifi_rssi_dbm WiFi signal strength from the AirGradient device perspective, in dBm
# TYPE airgradient_wifi_rssi_dbm gauge
# UNIT airgradient_wifi_rssi_dbm dbm
airgradient_wifi_rssi_dbm{} -51
# HELP airgradient_co2_ppm Carbon dioxide concentration as measured by the AirGradient S8 sensor, in parts per million
# TYPE airgradient_co2_ppm gauge
# UNIT airgradient_co2_ppm ppm
airgradient_co2_ppm{} 589
# HELP airgradient_pm1_ugm3 PM1.0 concentration as measured by the AirGradient PMS sensor, in micrograms per cubic meter
# TYPE airgradient_pm1_ugm3 gauge
# UNIT airgradient_pm1_ugm3 ugm3
airgradient_pm1_ugm3{} 3
# HELP airgradient_pm2d5_ugm3 PM2.5 concentration as measured by the AirGradient PMS sensor, in micrograms per cubic meter
# TYPE airgradient_pm2d5_ugm3 gauge
# UNIT airgradient_pm2d5_ugm3 ugm3
airgradient_pm2d5_ugm3{} 3
# HELP airgradient_pm10_ugm3 PM10 concentration as measured by the AirGradient PMS sensor, in micrograms per cubic meter
# TYPE airgradient_pm10_ugm3 gauge
# UNIT airgradient_pm10_ugm3 ugm3
airgradient_pm10_ugm3{} 3
# HELP airgradient_pm0d3_p100ml PM0.3 concentration as measured by the AirGradient PMS sensor, in number of particules per 100 milliliters
# TYPE airgradient_pm0d3_p100ml gauge
# UNIT airgradient_pm0d3_p100ml p100ml
airgradient_pm0d3_p100ml{} 594
# HELP airgradient_tvoc_index The processed Total Volatile Organic Compounds (TVOC) index as measured by the AirGradient SGP sensor
# TYPE airgradient_tvoc_index gauge
airgradient_tvoc_index{} 220
# HELP airgradient_tvoc_raw_index The raw input value to the Total Volatile Organic Compounds (TVOC) index as measured by the AirGradient SGP sensor
# TYPE airgradient_tvoc_raw_index gauge
airgradient_tvoc_raw_index{} 30801
# HELP airgradient_nox_index The processed Nitrous Oxide (NOx) index as measured by the AirGradient SGP sensor
# TYPE airgradient_nox_index gauge
airgradient_nox_index{} 1
# HELP airgradient_temperature_degc The ambient temperature as measured by the AirGradient SHT sensor, in degrees Celsius
# TYPE airgradient_temperature_degc gauge
# UNIT airgradient_temperature_degc degc
airgradient_temperature_degc{} 23.69
# HELP airgradient_humidity_percent The relative humidity as measured by the AirGradient SHT sensor
# TYPE airgradient_humidity_percent gauge
# UNIT airgradient_humidity_percent percent
airgradient_humidity_percent{} 39
# EOF