perf pmu: Support MetricExpr header in JSON event list

Add support for parsing the MetricExpr header in the JSON event lists
and storing them in the alias structure.

Used in the next patch.

v2: Change DividedBy to MetricExpr
v3: Really catch all uses of DividedBy

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170320201711.14142-10-andi@firstfloor.org


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