Update the default metrics.yml file
Update the default metrics.yml file

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

stackforge/cloudkitty,stackforge/cloudkitty,openstack/cloudkitty,openstack/cloudkitty