Add docs on the EventCounters emitted by ASP.NET Core

[shirhatti]

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.

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

[mkArtakMSFT]

@BrennanConroy who would be best positioned to document this? Please assign.

[cijothomas]

There is a good list already here, including many asp.net core ones: https://docs.microsoft.com/en-us/dotnet/core/diagnostics/event-counters#available-counters

[mkArtakMSFT]

@BrennanConroy given the above list @cijothomas has pointed out, should we close this issue?

[BrennanConroy]

That list is nice, but we've also been listing our counters on our docs as well. And from Sourabh's comment, we would like to go into a little bit of detail. https://docs.microsoft.com/aspnet/core/grpc/diagnostics?view=aspnetcore-5.0#metrics