Override help string

[llamafilm]

It would be helpful to override the help text, for MIBs that are either lacking or inaccurate.

The help string is defined here:

snmp_exporter/generator/tree.go

Line 353 in 9c42d6c

Help: n.Description + " - " + n.Oid,

I propose leaving that format as-is and allowing the user to override description in the generator.

This is especially useful in conjunction with the recent addition of scaling factor in #1026. For example, this Cyberpower UPS returns all sensor values as integers, so after scaling the voltage to the correct value, the help text is wrong. This can be fixed in snmp.yml but I think we need a way to do it in the generator.

  cyberpower_ups:
    walk:
      - 1.3.6.1.4.1.3808.1.1.1.3 # upsInput
    overrides:
      upsAdvanceInputLineVoltage:
        scale: 0.1

# HELP upsAdvanceInputLineVoltage The input line voltage in 1/10 VAC. - 1.3.6.1.4.1.3808.1.1.1.3.2.1
# TYPE upsAdvanceInputLineVoltage gauge
upsAdvanceInputLineVoltage 120.2

[SuperQ]

Yea, that's a great idea. The help text thing crossed my mind when I added the scale thing, but I forgot to write down a feature issue.