Add migration documentation mapping .NET Framework performance counters to .NET Core metrics

[noahfalk]

Documentation Request

.NET Framework had many Windows Performance Counters and customers want guidance for what to do when migrating. We should help them find our new counters, understand the mapping between old and new, and offer guidance on what to do for counters that have no equivalent.

Previous documentation

.NET Framework Performance Counters: https://learn.microsoft.com/en-us/dotnet/framework/debug-trace-profile/performance-counters .NET EventCounters: https://learn.microsoft.com/en-us/dotnet/core/diagnostics/available-counters