Update Metrics from 3.0.0-BETA1 to 3.0.0-BETA3

The package name for Metrics changed from com.yammer.metrics in
3.0.0-BETA1 to com.codahale.metrics in 3.0.0-BETA2 and later.

In addition to search-and-replacing the package name, this commit
changes MetricRegistry(String) constructor calls to no-arg constructor
calls.  The latest version of Metrics only has one MetricRegistry
constructor, a no-arg constructor.
Fix old MetricRegistry-related javadoc