Some enhancements to the JSON parser to prepare for metrics support

- Parse the new MetricGroup field
- Support JSON events with no event name, that have only MetricName.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170831194036.30146-3-andi@firstfloor.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>