Support metrics using System.Diagnostics.Metrics

With the release of .NET 6 last year, a new metrics API was introduced. This is available in System.Diagnostics.DiagnosticSource 6.0 package, which is backwards compatible with older .net runtimes (so we **do not** need to target .NET 6)

https://docs.microsoft.com/en-us/dotnet/core/diagnostics/metrics-instrumentation

We should use this API to emit metrics for select DTFx scenarios. Customers can then listen to these metrics themselves and export them out of process appropriately, or use an existing SDK like OpenTelemetry to export them.

We can start with building a list of metrics we want to collect, their names, value significance, and any dimensions.

Relies on #698