perf metricgroup: Factor out metricgroup__add_metric_weak_group()

perf metricgroup: Factor out metricgroup__add_metric_weak_group()

Factor out metricgroup__add_metric_weak_group() which add metrics into a
weak group. The change can improve code readability. Because following
patch will introduce a function which add standalone metrics.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Link: http://lore.kernel.org/lkml/1582581564-184429-3-git-send-email-kan.liang@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>