Add Metrics section to .NET Manual Instrumentation documentation

[jchoffmann]

The .NET Manual Instrumentation documentation only contains information about tracing. A section for manually collecting metrics (and different types of metrics) would be very helpful.

[kennykguo]

Please assign this issue to me, thanks

[svrnm]

@kennykguo if you want to tackle this issue, make sure that it works well with this ongoing PR: #3834, I can share more details what needs to be done if you are interested.

[theletterf]

We really need someone to take care of this one. +CC @Kielek