Add threadpool -stat

[davidfowl]

I'd like to see a summary of the queues without dumping the work items themselves:

Statistics:
              MT    Count    TotalSize Class Name
00007ff831723fb0        2          272 System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1+AsyncStateMachineBox`1[[Microsoft.AspNetCore.Connections.ConnectionContext, Microsoft.AspNetCore.Connections.Abstractions],[Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketConnectionListener+<AcceptAsync>d__13, Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets]]
00007ff83187dc08        1          320 System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1+AsyncStateMachineBox`1[[System.Threading.Tasks.VoidTaskResult, System.Private.CoreLib],[Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.Http2Connection+<ProcessRequestsAsync>d__62`1[[Microsoft.AspNetCore.Hosting.HostingApplication+Context, Microsoft.AspNetCore.Hosting]], Microsoft.AspNetCore.Server.Kestrel.Core]]
00007ff8316fab60       18          576 Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal.IOQueue
00007ff8316a3840       31         2976 System.Threading.TimerQueueTimer
00007ff8318b7a00      102        19584 System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1+AsyncStateMachineBox`1[[System.Threading.Tasks.VoidTaskResult, System.Private.CoreLib],[Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol+<ProcessRequests>d__217`1[[Microsoft.AspNetCore.Hosting.HostingApplication+Context, Microsoft.AspNetCore.Hosting]], Microsoft.AspNetCore.Server.Kestrel.Core]]
00007ff83172ef10     1030        32960 System.Threading.QueueUserWorkItemCallbackDefaultContext`1[[Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.Internal.SocketConnection, Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets]]
00007ff831637ff0      999       111888 Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Infrastructure.KestrelConnection
Total 2183 objects

cc @stephentoub