Fix exporter duration metrics
Fix exporter duration metrics

luxas/heapster,whuwxl/heapster,huangyuqi/heapster,burmanm/heapster,luxas/heapster,whuwxl/heapster,piosz/heapster,krzyzacy/heapster,Stackdriver/heapster,adfin/statster,bluebreezecf/heapster,krzyzacy/heapster,huangyuqi/heapster,piosz/heapster,Stackdriver/heapster,adfin/statster,bluebreezecf/heapster,burmanm/heapster