perf metricgroup: Factor out for-each function and move out printing

perf metricgroup: Factor out for-each function and move out printing

Factor metricgroup__for_each_metric into its own function handling
regular and sys metrics. Make the metric adding and printing code use
it, move the printing code into print-events files.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250710235126.1086011-6-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>