perf script: Change metric format to use json metrics

perf script: Change metric format to use json metrics

The metric format option isn't properly supported. This change
improves that by making the sample events update the counts of an
evsel, where the shadow metric code expects to read the values.  To
support printing metrics, metrics need to be found. This is done on
the first attempt to print a metric. Every metric is parsed and then
the evsels in the metric's evlist compared to those in perf script
using the perf_event_attr type and config. If the metric matches then
it is added for printing. As an event in the perf script's evlist may
have >1 metric id, or different leader for aggregation, the first
metric matched will be displayed in those cases.

An example use is:
```
$ perf record -a -e '{instructions,cpu-cycles}:S' -a -- sleep 1
$ perf script -F period,metric
...
     867817
         metric:    0.30  insn per cycle
     125394
         metric:    0.04  insn per cycle
     313516
         metric:    0.11  insn per cycle
         metric:    1.00  insn per cycle
```

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