Work items:
- Added "user_id" to default groupby attributes
- Replaced "flavor" metadata by "flavor_name" for instances (it is the
default attribute name in gnocchi)
- Replaced network.incoming/outgoing.bytes by
network.incoming/outgoing.bytes.rate. The previous metrics were cumulative,
whereas the new ones are rates.
Change-Id: I55271bfbdd5a3457bea249dbefefd93a78ee057e