perf metrics: Remove the "No_group" metric group

Rather than place metrics without a metric group in "No_group" place
them in a a metric group that is their name. Still allow such metrics
to be selected if "No_group" is passed, this change just impacts perf
list.

Reviewed-by: Kan Liang <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/metricgroup.c

···
 	const char *g;
 	char *omg, *mg;
 
-	mg = strdup(pm->metric_group ?: "No_group");
+	mg = strdup(pm->metric_group ?: pm->metric_name);
 	if (!mg)
 		return -ENOMEM;
 	omg = mg;
···
 		if (strlen(g))
 			me = mep_lookup(groups, g, pm->metric_name);
 		else
-			me = mep_lookup(groups, "No_group", pm->metric_name);
+			me = mep_lookup(groups, pm->metric_name, pm->metric_name);
 
 		if (me) {
 			me->metric_desc = pm->desc;