Comment 2 for bug 1331508

@Lena

i believe the use case i was originally thinking of was to get the cpu_util value grouped by instance (which is a metadata value).

ie. something similar to the following (the sql below is most likely invalid):
select meter.cpu_util
from meter
join sample on meter.id = sample.meter_id
left outer join metadata_text on metadata_text.id = sample.resource_id
where metadata_text.key = 'instance_id'
groupby metadata_text.value

similarly, the original use case for the instance:<type> meter was because we could not build queries using the metadata. see:
https://review.openstack.org/#/c/40234/