feat(collectors): Add Open-Hardware Monitor collector

[zalgonoise]

Hi, this PR adds a collector for a running Open-Hardware Monitor service.

The idea of bringing this to windows_exporter spawned in #816 where the thermalzone collector was raising issues. Personally, I also tried to explore what could I collect using Windows WMI and CMI but to no success -- also being the first time exploring these Windows APIs (they are not pretty, in Go).

Since that didn't work out and someone had suggested a plan-B using Open-Hardware Monitor, that exposes its metrics via WMI, so that is what I went for with this PR.

It works perfectly well after a few weeks of usage, and I wanted to share it with the rest of the Prometheus community as well. Any feedback is appreciated.

[jkroepke]

Hey @zalgonoise

thanks for your PR.

I saw you try-hard invoke WMI on your own. That must be painfull. I recommend to take a look at the printer collector where the bultin capabilites of the exporter are used to invoke wmi.

Outside of the exporter, take a look at https://github.com/microsoft/wmi

Even Microsoft provide bindings for WMI.

[zalgonoise]

thanks, @jkroepke 😄

You're right, I eventually ended up trying the WMI calls through powershell (also a first time for that, since I rarely use Windows) so I was trying to reproduce the same calls in Go, without the proper context of how to do it properly.

I will set this PR in draft while I check your references and update my branch, then publish it again for a new review 😄 thanks a lot for your insight!!

[jkroepke]

Suggested change

	# ohwm collector
	# open_hardware_monitor collector

Just name it like that. By full name. People may better understand what it does and people google for it, they may find this. No one knows what ohwm could be unless they read the docs.

It would be great to have some small docs about how I can install the Open Hardware Monitor

[jkroepke]

@zalgonoise I ask myself, it's its an good idea to introduce a collector for an program which does not get maintenance anymore: https://github.com/openhardwaremonitor/openhardwaremonitor