Expose global CF metrics

Patch by Chris Lohfink, reviewed by brandonwilliams for CASSANDRA-7273
Merge branch 'cassandra-1.2' into cassandra-2.0
Expose global CF metrics

Patch by Chris Lohfink, reviewed by brandonwilliams for CASSANDRA-7273
Merge branch 'cassandra-2.0' into cassandra-2.1
Expose global CF metrics

Patch by Chris Lohfink, reviewed by brandonwilliams for CASSANDRA-7273