pmdaperfevent: add perfevent.derived.active metric
pmdaperfevent: add perfevent.derived.active metric

This patch adds a metric perfevent.derived.active which will be
permanently present and gives the number of active perf derived
events.  This also helps in avoiding a warning message which is
shown if there are no perf derived events.

Signed-off-by: Hemant Kumar <7123f6a6628a9fc84f20db1d8af7bbec27c94c78@linux.vnet.ibm.com>

wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp