Comment 4 for bug 1391778

namespace will be removed. and duplicate entry will be ignored, so from your settings, you will get {'': 'localhost'}

if you can set the resource's metadata, you can do the following:
set reserved_metadata_namespace=metering.
then if resource has metadata: {'metering.project_cos': 'dev', 'metering.fqdn': 'localhost'}
the final sample's resource_metadata can have user_metadata.project_cos: dev and user_metadata.fqdn: localhost

for more doc, read http://docs.openstack.org/developer/ceilometer/measurements.html#user-defined-sample-metadata-for-nova