Add discovery of Performance Counter instances on Windows Agent

I'd like to be able to discover Performance Counter instance natively via the Windows Agent. This way Discovery Rules can be created for the automated monitoring of Multi-Instance Performance Counters such as Process, PhysicalDisk, LogicalDisk, Paging File, etc.

I initially used a PowerShell script to discover Perf Counter instances but realized most of the work is already done in the Agent source and is a much preferred method.

The following code has worked nicely for me (Agent sources v2.2.1)

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