perf metricgroup: Add care to picking the evsel for displaying a metric

perf metricgroup: Add care to picking the evsel for displaying a metric

Rather than using the first evsel in the matched events, try to find
the least shared non-tool evsel. The aim is to pick the first evsel
that typifies the metric within the list of metrics.

This addresses an issue where Default metric group metrics may lose
their counter value due to how the stat displaying hides counters for
default event/metric output.

For a metricgroup like TopdownL1 on an Intel Alderlake the change is,
before there are 4 events with metrics:
```
$ perf stat -M topdownL1 -a sleep 1

 Performance counter stats for 'system wide':

     7,782,334,296      cpu_core/TOPDOWN.SLOTS/          #     10.4 %  tma_bad_speculation
                                                  #     19.7 %  tma_frontend_bound
     2,668,927,977      cpu_core/topdown-retiring/       #     35.7 %  tma_backend_bound
                                                  #     34.1 %  tma_retiring
       803,623,987      cpu_core/topdown-bad-spec/
       167,514,386      cpu_core/topdown-heavy-ops/
     1,555,265,776      cpu_core/topdown-fe-bound/
     2,792,733,013      cpu_core/topdown-be-bound/
       279,769,310      cpu_atom/TOPDOWN_RETIRING.ALL/   #     12.2 %  tma_retiring
                                                  #     15.1 %  tma_bad_speculation
       457,917,232      cpu_atom/CPU_CLK_UNHALTED.CORE/  #     38.4 %  tma_backend_bound
                                                  #     34.2 %  tma_frontend_bound
       783,519,226      cpu_atom/TOPDOWN_FE_BOUND.ALL/
        10,790,192      cpu_core/INT_MISC.UOP_DROPPING/
       879,845,633      cpu_atom/TOPDOWN_BE_BOUND.ALL/
```

After there are 6 events with metrics:
```
$ perf stat -M topdownL1 -a sleep 1

 Performance counter stats for 'system wide':

     2,377,551,258      cpu_core/TOPDOWN.SLOTS/          #      7.9 %  tma_bad_speculation
                                                  #     36.4 %  tma_frontend_bound
       480,791,142      cpu_core/topdown-retiring/       #     35.5 %  tma_backend_bound
       186,323,991      cpu_core/topdown-bad-spec/
        65,070,590      cpu_core/topdown-heavy-ops/      #     20.1 %  tma_retiring
       871,733,444      cpu_core/topdown-fe-bound/
       848,286,598      cpu_core/topdown-be-bound/
       260,936,456      cpu_atom/TOPDOWN_RETIRING.ALL/   #     12.4 %  tma_retiring
                                                  #     17.6 %  tma_bad_speculation
       419,576,513      cpu_atom/CPU_CLK_UNHALTED.CORE/
       797,132,597      cpu_atom/TOPDOWN_FE_BOUND.ALL/   #     38.0 %  tma_frontend_bound
         3,055,447      cpu_core/INT_MISC.UOP_DROPPING/
       671,014,164      cpu_atom/TOPDOWN_BE_BOUND.ALL/   #     32.0 %  tma_backend_bound
```

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>