perf list: Display the PMU name associated with a perf metric in JSON

The 'perf stat --cputype' option can be used to filter which metrics
will be applied, for this reason the JSON metrics have an associated
PMU.

List this PMU name in the 'perf list' output in JSON mode so that
tooling may access it.

An example of the new field is:
```
{
        "MetricGroup": "Backend",
        "MetricName": "tma_core_bound",
        "MetricExpr": "max(0, tma_backend_bound - tma_memory_bound)",
        "MetricThreshold": "tma_core_bound > 0.1 & tma_backend_bound > 0.2",
        "ScaleUnit": "100%",
        "BriefDescription": "This metric represents fraction of slots where ...
        "PublicDescription": "This metric represents fraction of slots where ...
        "Unit": "cpu_core"
},
```

Signed-off-by: Ian Rogers <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Weilin Wang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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