Document the unit for every metric in the `Metric` enum

[tiyash-basu-frequenz]

What's needed?

The Metric enum variants do not have their units documented. This will cause confusions and bugs.

For reference, we stick to using base SI units for these metrics. So,

• voltage: V
• current: A
• DC/AC-active power: W
• AC-reactive power: VAr
• AC-apparent power: VA
• energy: Wh, VArh, VAh

Proposed solution

We need to document every variant here, and specify their units.

Use cases

No response

Alternatives and workarounds

Until this issue is resolved, the units can be referred to from electrical.proto. While the data structures in this file are very different from those in metric_sample.proto, the units of the metrics are unchanged.

Additional context

No response

[llucax]

I guess:

• Frequency: Hz
• Temperature: °C
• SoC: %
• Battery Capacity -> Energy
• Factor: no unit

?

What about?

• Speed: km/h? m/s?
• Direction: degrees with some convention? radians?
• Irrandiance: ?
• Air pressure: ?
• Dew point -> Temprature?
• Relative humidity: %?
• Harmonic distortion: ?

[cwasicki]

For the meteorological units these are probably most common:

• Wind speed: m/s
• Wind direction: degrees (0-360) with North = 0
• Irradiance: W/m**2
• Dew point temperature: degree Celsius
• Relative humidity: % (0-100)
• Air pressure: hPa

Not fully sure about THD, maybe as a percentage 0-100 @tiyash-basu-frequenz?

[tiyash-basu-frequenz]

THD is a ratio, so it does not have units.

[cwasicki]

Sure, but ratios can be expressed in percent, which seems to be the case for THD too (besides dB): https://en.wikipedia.org/wiki/Total_harmonic_distortion.