perf stat: Fix adding multiple event groups

The -M metric group parser threw away the events of earlier groups when
multiple groups were specified. Fix this here by not overwriting the
string incorrectly.

Now this works correctly:

% perf stat -M Summary,SMT --metric-only -a sleep 1

 Performance counter stats for 'system wide':

Instructions CPI CLKS         CPU_Utilization GFLOPs SMT_2T_Utilization SMT_2T_Utilization Kernel_Utilization CoreIPC CORE_CLKS
900907376.0  2.7 2398954144.0 0.1             0.0    0.2                0.2                0.1                0.4     2080822855.5

while previously it would only show the SMT metrics.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170914205735.18431-1-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/metricgroup.c

@@ -373,9 +373,6 @@ static int metricgroup__add_metric(const char *metric, struct strbuf *events,
 	int ret = -EINVAL;
 	int i, j;
 
-	strbuf_init(events, 100);
-	strbuf_addf(events, "%s", "");
-
 	if (!map)
 		return 0;
 
@@ -433,6 +430,10 @@ static int metricgroup__add_metric_list(const char *list, struct strbuf *events,
 	if (!nlist)
 		return -ENOMEM;
 	llist = nlist;
+
+	strbuf_init(events, 100);
+	strbuf_addf(events, "%s", "");
+
 	while ((p = strsep(&llist, ",")) != NULL) {
 		ret = metricgroup__add_metric(p, events, group_list);
 		if (ret == -EINVAL) {