osd/scrub: performance counters: count I/Os, use unlabeled counters
Reviewed-by: Alex Ainscow <aainscow@uk.ibm.com> Reviewed-by: Radoslaw Zarzynski <rzarzyns@redhat.com> Reviewed-by: Bill Scales <bill_scales@uk.ibm.com> Reviewed-by: Samuel Just <sjust@redhat.com>