expose global ColumnFamily metrics

It would be very useful to have cassandra expose ColumnFamily metrics that span all column families. A general purpose cassandra monitoring system built up around the current ColumnFamily metrics really only has a couple of choices right now: publish metrics for all column families or fetch metrics for all column families, aggregate them and then publish the aggregated metrics. The first can be quite expensive for the downstream monitoring system and the second is a piece of work that it seems is better pushed into cassandra itself.

Perhaps these global ColumnFamily metrics could be published under a name of:

org.apache.cassandra.metrics:type=(ColumnFamily|IndexColumnFamily),name=(Metric name)

(Same as existing ColumnFamily metrics, but with no keyspace or scope.)