Add remove(String key, Metric metric) method to MetricRegistry interface

Add a `remove(String name, Metric metric)` method to the `MetricRegistry` interface. Right now the interface only contains a `remove(String name)` method.

This additional remove method will give users the power to remove a `Metric` with the specified `name` from the metric registry if and only if the provided `metric` matches the object in the registry.

Implementing the new `remove(String name, Metric metric)` should be straight forward because the `MetricRegistryImpl` class stores metrics in a `ConcurrentMap`, which already contains a `remove(Object key, Object value)` method

This change will not be a breaking one because the interface is marked with `@IntefaceStability.Evolving`

rmdmattingly