perf jevents: Move json encoding to its own functions
Have dedicated encode functions rather than having them embedded in
MetricGroup. This is to provide some uniformity in the Metric ToXXX
routines.
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Falcon <thomas.falcon@intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
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