perf ftrace latency: allow to hide empty buckets

Especially while using several buckets, it isn't uncommon to have some
of them empty and reading the histogram may be a bit more complex:

  # perf ftrace latency -a -T mutex_lock --bucket-range 5 --max-latency 200
  #   DURATION     |  COUNT | GRAPH                                  |
       0 -    5 us |  14816 | ###################################### |
       5 -   10 us |   1228 | ###                                    |
      10 -   15 us |    438 | #                                      |
      15 -   20 us |    106 |                                        |
      20 -   25 us |     21 |                                        |
      25 -   30 us |     11 |                                        |
      30 -   35 us |      1 |                                        |
      35 -   40 us |      2 |                                        |
      40 -   45 us |      4 |                                        |
      45 -   50 us |      0 |                                        |
      50 -   55 us |      1 |                                        |
      55 -   60 us |      0 |                                        |
      60 -   65 us |      1 |                                        |
      65 -   70 us |      1 |                                        |
      70 -   75 us |      1 |                                        |
      75 -   80 us |      2 |                                        |
      80 -   85 us |      0 |                                        |
      85 -   90 us |      1 |                                        |
      90 -   95 us |      0 |                                        |
      95 -  100 us |      1 |                                        |
     100 -  105 us |      0 |                                        |
     105 -  110 us |      0 |                                        |
     110 -  115 us |      0 |                                        |
     115 -  120 us |      0 |                                        |
     120 -  125 us |      1 |                                        |
     125 -  130 us |      0 |                                        |
     130 -  135 us |      0 |                                        |
     135 -  140 us |      1 |                                        |
     140 -  145 us |      0 |                                        |
     145 -  150 us |      0 |                                        |
     150 -  155 us |      0 |                                        |
     155 -  160 us |      0 |                                        |
     160 -  165 us |      0 |                                        |
     165 -  170 us |      0 |                                        |
     170 -  175 us |      0 |                                        |
     175 -  180 us |      0 |                                        |
     180 -  185 us |      0 |                                        |
     185 -  190 us |      0 |                                        |
     190 -  195 us |      0 |                                        |
     195 -  200 us |      0 |                                        |
     200 -  ... us |      2 |                                        |

Allow the optional flag --hide-empty to remove buckets with no element
and produce a more compact graph. This feature could be misleading since
there is no clear indication for missing buckets, for this reason it's
disabled by default.

  # perf ftrace latency -a -T mutex_lock --bucket-range 5 --max-latency --hide-empty 200
  #   DURATION     |  COUNT | GRAPH                                  |
       0 -    5 us |  14816 | ###################################### |
       5 -   10 us |   1228 | ###                                    |
      10 -   15 us |    438 | #                                      |
      15 -   20 us |    106 |                                        |
      20 -   25 us |     21 |                                        |
      25 -   30 us |     11 |                                        |
      30 -   35 us |      1 |                                        |
      35 -   40 us |      2 |                                        |
      40 -   45 us |      4 |                                        |
      50 -   55 us |      1 |                                        |
      60 -   65 us |      1 |                                        |
      65 -   70 us |      1 |                                        |
      70 -   75 us |      1 |                                        |
      75 -   80 us |      2 |                                        |
      85 -   90 us |      1 |                                        |
      95 -  100 us |      1 |                                        |
     120 -  125 us |      1 |                                        |
     135 -  140 us |      1 |                                        |
     200 -  ... us |      2 |                                        |

Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20250207080446.77630-2-gmonaco@redhat.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>