Implement a statistics HTTP server

If you don't want this class to be assignable, the way to do it is not to allow someone to assign it and then die at runtime - perhaps a long time after they wrote the incorrect code.  

The way to do it is to forbid the assignment operator so there's a compile-time error that they immediately have to fix.

In C++11, you write in the declaration of this method in the .h file: `MetricsCounterImpl& operator= (MetricsCounterImpl const&) = delete;` - super-neat!