[SQLalchemy] Add groupby ability resource_metadata

It would be useful to have groupby functionality to derive meters
by their resource_metadata. This commit adds groupby by
resource_metadata.instance_type for sqlalchemy.

Change-Id: I568b5ca3d84371c4fd3bbdfed7b3e801164dbc5e
Partially-Closes-Bug: 1331508
Merge "[SQLalchemy] Add groupby ability resource_metadata"