An event group is a critical relationship. There is a -g option that can
display the relationship. But it's hard for a user to know when should
this option be applied.

If there is an event group in the perf record, print a hint to suggest
the user apply the -g to display the group information.

With the patch,

  $ perf record -e "{cycles,instructions},instructions" sleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.024 MB perf.data (4 samples) ]
  $

  $ perf evlist
  cycles
  instructions
  instructions
  # Tip: use 'perf evlist -g' to show group information

  $ perf evlist -g
  {cycles,instructions}
  instructions
  $

Committer testing:

So for a perf.data file _with_ a group:

  root@number:~# perf evlist -g
  {cpu_core/branch-instructions/pp,cpu_core/branches/}
  dummy:u
  root@number:~# perf evlist
  cpu_core/branch-instructions/pp
  cpu_core/branches/
  dummy:u
  # Tip: use 'perf evlist -g' to show group information
  root@number:~#

Then for something _without_ a group, no hint:

  root@number:~# perf record ls
  <SNIP>
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.035 MB perf.data (7 samples) ]
  root@number:~# perf evlist
  cpu_atom/cycles/P
  cpu_core/cycles/P
  dummy:u
  root@number:~#

No suggestion, good.
Suggested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Closes: https://lore.kernel.org/lkml/ZttgvduaKsVn1r4p@x1/
Link: https://lore.kernel.org/r/20240908202847.176280-1-kan.liang@linux.intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>