Track timer count and add Timer.ActiveCount (dotnet/coreclr#25061)
Track timer count and add Timer.ActiveCount (dotnet/coreclr#25061)

* Track timer count and add Timer.Count

Part of https://github.com/dotnet/corefx/issues/38422

* Use lock to prevent tearing

* Add assert

* Rename Count to ActiveCount

* Fix build

Signed-off-by: dotnet-bot <03c4cb6107ae99956a7681f2c97ff7104809a2bd@microsoft.com>

ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,wtgodbe/corefx,wtgodbe/corefx,wtgodbe/corefx,BrennanConroy/corefx,shimingsg/corefx,shimingsg/corefx,wtgodbe/corefx,ViktorHofer/corefx,ViktorHofer/corefx,shimingsg/corefx,ericstj/corefx,ericstj/corefx,wtgodbe/corefx,wtgodbe/corefx,ericstj/corefx,BrennanConroy/corefx,BrennanConroy/corefx,ViktorHofer/corefx,ViktorHofer/corefx,shimingsg/corefx,ViktorHofer/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx