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