Add support for entPhySensorTable values about snmp_exporter HOT 3 OPEN

I think it's something we could do in code, but it would need to be generic enough to work with other MIBs.

I don't have time to implement this, but PRs would be welcome.

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We just recently added a scale configuration option to the code. In theory, we could have an override that looks up the scale factor from another OID.

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My workaround is not too bad. The downsides are that it moves some logic into prometheus' recording rules and that it adds a little too much data to the database that I don't really want.

generator.yml

modules:
  entity-physical-sensors:
    walk:
      - entPhysicalIndex
      - entPhysicalDescr
      - entPhySensorScale
      - entPhySensorPrecision
      - entPhySensorValue
    lookups:
      - source_indexes: [entPhysicalIndex]
        lookup: entPhysicalDescr
    overrides:
      entPhysicalIndex: {ignore: true}
      entPhysicalDescr: {ignore: true}

prometheus/rules-recording.yml

groups:
  - name: snmp-exporter
    rules:
      - record: entPhySensorValueScaled
        expr: entPhySensorValue * 10 ^ (3 * (entPhySensorScale - 9) - entPhySensorPrecision)

@SuperQ yes, I noticed that new static field. But I would need something to handle the formula valueScaled = value * 10 ^ (3 * (scale - 9) - precision). You do a static scale, but I would need to be able to compose add, subtract, multiply, and pow. Which is a lot of new functionality. That, or add a special case for entPhySensorTable.

I'm content with a "no, that's out of scope." But I thought I'd ask just in case.

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