When we have events for each cgroup, the metric should be printed for
each cgroup separately.  Add print_cgroup_counter() to handle that
situation properly.

Also change print_metric_headers() not to print duplicate headers
by checking cgroups.

  $ perf stat -a --for-each-cgroup system.slice,user.slice --metric-only sleep 1

   Performance counter stats for 'system wide':

                                     GHz       insn per cycle branch-misses of all branches
   system.slice                   3.792                0.61                                3.24%
   user.slice                     3.661                2.32                                0.37%

         1.016111516 seconds time elapsed
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20221114230227.1255976-19-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>