perf shell completion: Support completion of metrics/metricgroups

Allow metrics to expand for -M or --metrics options.

Committer testing:

  # grep -m1 'model name' /proc/cpuinfo
  model name	: AMD Ryzen 9 5950X 16-Core Processor
  #

Before:

  Just expansion of files/directories in the pwd are expanded:

  # . tools/perf/perf-completion.sh
  # perf stat -M b
  block/ build/
  # perf stat -M b

After:

  # . tools/perf/perf-completion.sh
  # perf stat -M
  all_l2_cache_accesses  all_remote_links_outbound   data_fabric          l1_itlb_misses                  l2_cache_misses_from_l2_hwpf  macro_ops_dispatched       tlb
  all_l2_cache_hits      branch_misprediction_ratio  decoder              l2_cache                        l3_cache                      nps1_die_to_dram
  all_l2_cache_misses    branch_prediction           ic_fetch_miss_ratio  l2_cache_accesses_from_l2_hwpf  l3_read_miss_latency          op_cache_fetch_miss_ratio
  # perf stat -M branch_
  branch_misprediction_ratio  branch_prediction
  # perf stat -M branch_prediction -a sleep 1

   Performance counter stats for 'system wide':

       115,079,765      ex_ret_brn                       #      4.0 %  branch_misprediction_ratio
         4,561,456      ex_ret_brn_misp

       1.015925106 seconds time elapsed

  #
Suggested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20230905181554.3202873-3-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/perf-completion.sh

@@ -202,6 +202,10 @@ __perf_main ()
 		$prev_skip_opts == @(record|stat|top) ]]; then
 	        local evts=$($cmd list --raw-dump pfm)
 		__perfcomp "$evts" "$cur"
+	elif [[ $prev == @("-M"|"--metrics") &&
+		$prev_skip_opts == @(stat) ]]; then
+	        local metrics=$($cmd list --raw-dump metric metricgroup)
+		__perfcomp "$metrics" "$cur"
 	else
 		# List subcommands for perf commands
 		if [[ $prev_skip_opts == @(kvm|kmem|mem|lock|sched|