Add dhcp collector

[breed808]

Unfortunately perflib does not expose scope statistics
(DhcpServerv4ScopeStatistics), and WMI queries to this class don't
appear to be possible with the stackexchange/wmi module.

[carlpett]

Thanks @breed808! Some minor comments, but looks good as usual :)

[carlpett]

[...] and WMI queries to this class don't appear to be possible with the stackexchange/wmi module.

Is it a namespaced class, or something more exotic? We do namespaced queries in the process/iis collector:
https://github.com/martinlindhe/wmi_exporter/blob/91a64fecb850fae0a90219bfcd045c3bc1838cc7/collector/process.go#L189

[breed808]

It's been a while since I've looked at the WMI class for the scope statistics. Would it be preferable to investigate the class as a separate collector, to keep the Perflib & WMI components separate?

[carlpett]

It's been a while since I've looked at the WMI class for the scope statistics. Would it be preferable to investigate the class as a separate collector, to keep the Perflib & WMI components separate?

I think we'd rather have it in a single collector, but it might make sense to split it in different collect-functions as we've done in some places.

[breed808]

I think most of the issues have been ironed out by now, I just need to investigate the WMI class. Will update when this has been done.

[breed808]

I've looked into the WMI class by testing the wmi library directly. However, this has not returned any data. Running Get-WmiObject -namespace "root\Microsoft\Windows\DHCP" -query 'SELECT * FROM DhcpServerv4ScopeStatistics' also returns no data.

I can see someone else has had this issue but the general advice appears to be "Use the Get-DhcpServerv4ScopeStatistics PowerShell command instead of querying WMI directly".

I'm not sure how this can be queried with WMI 😞

[carlpett]

Okay, let's skip the scope statistics for now and it can be added later if we figure out how. Just one minor doc nit, then we I think we're done.
Great work! 🎉