Avoid race condition in DiagnosticCounter (dotnet/coreclr#26260)

Avoid race condition in DiagnosticCounter (dotnet/coreclr#26260)

* Avoid race condition in DiagnosticCounter

If while a DiagnosticCounter instance is being created the timer fires to process counters, the timer may end up evaluating a DiagnosticCounter whose constructor or derived types' constructor hasn't finished configuring the instance's state yet.

* Remove unnecessary #if

Commit migrated from https://github.com/dotnet/coreclr/commit/5ae4f6d650b001f61961316206b373c9c71e5fe5