[System] Add performanceCounters element to System.Diagnostics app.config configuration section

See [MSDN](http://msdn.microsoft.com/en-us/library/ms229387(v=vs.110).aspx) for details about the element.
The value that is set by the element isn't currently used in Mono, but applications using it in their app.config crash otherwise (e.g. http://stackoverflow.com/questions/25741651).
Yeah, I just copied the `AddAssertNode` from above and the other `Add*` methods have `private` too, so I chose to be consistent. Should I change it?