[Azure] [Docs] Expand Azure VM Guest Metrics documentation

[zmoog]

The Azure Virtual Machines Metrics documentation contains a section about the Guest Metrics.

It is great it's here, but I believe we should expand it a little and make it more actionable for the users, like:

• Which section of the Azure Portal to visit
• Step by step instructions of what to enable
• Link to pages with additional information

Relates:

• [Azure][compute_vm][compute_vm_scaleset] metrics Azure.VM.Windows.GuestMetrics and Azure.VM.Linux.GuestMetrics seems to be not available #7167

[alaudazzi]

@zmoog
As discussed on Friday Dec 1, here is a draft with the doc fix. As soon as you give me your 👍I'll push a doc PR.