Filter low-value generic IPMI sensor metrics

[SilenceAdele]

• For native IPMI implementation:
  • Only export ipmi_sensor_value when the sensor has a real physical unit (i.e. SensorUnitType != Unspecified)
  • Only export ipmi_sensor_state when the mapped state is not NaN.
• For freeipmi implementation:
  • Only export ipmi_sensor_value/ipmi_sensor_state when not NaN.

[SilenceAdele]

Related explanation: #312 (comment)

[bitfehler]

I would be open to dropping values that are NaN, but it would have to be opt-in, e.g. a command line switch. Dropping based on an unspecified unit seems dangerous, though, I don't think that makes sense.

Also, a word of caution: I took a closer look at the outputs of the native collectors. I have to say: I would not use that for production. FreeIPMI does a lot of post-processing of the raw sensor data, go-ipmi does not, and reproducing that to the effect that both have the same semantics is a massive undertaking...

[SuperQ]

I agree that we should have some flags for this, but maybe opt-out is probably better for most use cases.

[SilenceAdele]

Thanks for your suggestions! @bitfehler @SuperQ

I've introduced an opt-in flag --filter-nan-sensors to filter out generic sensor metrics with NaN values.
This keeps the existing behavior unchanged by default, while providing flexibility for users who prefer to exclude NaN metrics.

Happy to refine this further if you have any concerns.