Add docs on the EventCounters emitted by ASP.NET Core

Starting in 3.0, we added EventCounters to ASP.NET Core. We should document what counters are emitted and how to observe them (using the dotnet-counters tool and Visual Studio).

We now have EventCounters emitted by Hosting, SignalR, gRPC, and [Kestrel](https://github.com/dotnet/aspnetcore/pull/21649).

If possible, we should also some heuristics of what the expected value of each of these counters is and how they are expected to diverge during abnormal conditions. Especially given that some of these counters have seemingly confusing names. For example, `connection-queue-length` in Kestrel means something completely different than what it did in `System.Web` because of differences in architecture.

cc @davidfowl