perf stat: Factor out generic metric printing

The 'perf stat' shadow metric printing already supports generic metrics.
Factor out the code doing that into a separate function that can be
re-used in a later patch.

No behavior changes.

v2: Fix indentation

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-4-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>