perf stat: Fix no metric header if --per-socket and --metric-only set

We received a report that was no metric header displayed if --per-socket
and --metric-only were both set.

It's hard for script to parse the perf-stat output. This patch fixes this
issue.

Before:

  root@kbl-ppc:~# perf stat -a -M CPI --metric-only --per-socket
  ^C
   Performance counter stats for 'system wide':

  S0        8                  2.6

         2.215270071 seconds time elapsed

  root@kbl-ppc:~# perf stat -a -M CPI --metric-only --per-socket -I1000
  #           time socket cpus
       1.000411692 S0        8                  2.2
       2.001547952 S0        8                  3.4
       3.002446511 S0        8                  3.4
       4.003346157 S0        8                  4.0
       5.004245736 S0        8                  0.3

After:

  root@kbl-ppc:~# perf stat -a -M CPI --metric-only --per-socket
  ^C
   Performance counter stats for 'system wide':

                               CPI
  S0        8                  2.1

         1.813579830 seconds time elapsed

  root@kbl-ppc:~# perf stat -a -M CPI --metric-only --per-socket -I1000
  #           time socket cpus                  CPI
       1.000415122 S0        8                  3.2
       2.001630051 S0        8                  2.9
       3.002612278 S0        8                  4.3
       4.003523594 S0        8                  3.0
       5.004504256 S0        8                  3.7
Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200331180226.25915-1-yao.jin@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/stat-shadow.c

@@ -803,8 +803,11 @@ static void generic_metric(struct perf_stat_config *config,
 				     out->force_header ?
 				     (metric_name ? metric_name : name) : "", 0);
 		}
-	} else
-		print_metric(config, ctxp, NULL, NULL, "", 0);
+	} else {
+		print_metric(config, ctxp, NULL, NULL,
+			     out->force_header ?
+			     (metric_name ? metric_name : name) : "", 0);
+	}
 
 	for (i = 1; i < pctx.num_ids; i++)
 		zfree(&pctx.ids[i].name);