perf metrics: Remove the "No_group" metric group

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 <kan.liang@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240403164636.3429091-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>