Fix problem where value_meta is null in JSON
Fix problem where value_meta is null in JSON

Change-Id: Ie6d88227a9c2618e909e5ff5ead15fe997824215

stackforge/monasca-persister,stackforge/monasca-persister,stackforge/monasca-persister,openstack/monasca-persister,openstack/monasca-persister,sapcc/monasca-persister,openstack/monasca-persister