[MongoDB] Add groupby ability on 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 mongodb.

Change-Id: I26e7fb4fa2835d283cb6c83e0aec2dba4ac9bf00
Partially-Closes-Bug: 1331508
Merge "[MongoDB] Add groupby ability on resource_metadata"