[metrics-user] registerAll with metric sets should take a prefix

Right now I'm registering three metric sets:

        metricRegistry.registerAll( new BufferPoolMetricSet( mBeanServer ) );

        metricRegistry.registerAll( new ThreadStatesGaugeSet() );

        metricRegistry.registerAll( new MemoryUsageGaugeSet() );

... right now this yields metrics named:

    "total.init" : {
      "value" : 91422720
    },

What the heck is total.init!? :-P

I have no idea what this means.. 

So.. first, I think they should either all have some sort of reasonable prefix, like ,

jvm.threads.foo.bar 

... instead of just foo.bar

of the user should be able to specify it. 

Guessing you probably found this already and the API could be a little clearer here but if you use just MetricRegistry.register(name, Metric) you can provide a prefix and your can pass a MetricSet (as a MetricSet implements the Metric interface) - there's some logic to then use the provided name as a prefix for the MetricSet here - https://github.com/dropwizard/metrics/blob/master/metrics-core/src/main/java/com/codahale/metrics/MetricRegistry.java#L85

Even so, there is no explicit representation of a MetricSet in the MetricRegistry (or, alternatively, having Metric objects optionally specifying a parent MetricSet name).

If this existed, reporters and the MetricsServlet would have the chance to group these metrics to their clients, e.g. a JmxReporter could be able to register a set of related Gauges into a CompositeData object.
However, this looks like an intervention to Metrics' core API, which might be acceptable for 4.0 but probably only if there is a common demand for it... 

Any opinions? Ryan?