Docs/test updates/migration guidance for new runtime metrics in dotnet-counters

[noahfalk]

We recently added new runtime metrics based on the Meter API to .NET. Although the previous EventCounter-based runtime metrics are still present, these new metrics are intended to be the long-term path forward. In order to complete the scenario we need to do several followup tasks:

1. We've got dotnet-counters tests that explicitly validated against the old metric names so we should update those tests. Example ( DotnetCounters.UnitTests.CounterMonitorPayloadTests.TestCounterMonitorSystemRuntimeMetricsCSV).
2. We need to add docs about the new metrics to our official docs and guidance for anyone migrating.
3. We should add some mechanism to dotnet-counters for folks to access the previous EventCounter based metrics if they still want to see them.

fyi @tarekgh @stevejgordon @dotnet/dotnet-monitor

[mikem8361]

TestCounterMonitorSystemRuntimeMetricsJSON and TestCounterMonitorSystemRuntimeMetricsCSV in CounterMonitorPayloadTests.cs need to be re-enabled after the test is fixed for recent 9.0 changes while maintaining backwards compatibility with .NET 6/7/8 .

[noahfalk]

fyi @samsp-msft

[noahfalk]

Everything was addressed, PRs linked in the description.