Document that creating timers and gauges with same tag cycles causes endless memory cycles

[robskillington]

[Raynos]

Actually, every counter, gauge and subscope allocated is another iteration in this loop ( https://github.com/uber-go/tally/blob/master/scope.go#L157 ).

The array of subscopes is unbounded if sub scopes are allocated dynamically and there is no way of removing things from the scope registry.

[robskillington]

https://github.com/uber-go/tally/pull/25 addresses this.

And yes, there is no way to unregister a scope currently. We could probably add a method to Scope for that but it does widen the breadth of the API for someone who is implementing Scope themselves.