Add example with return values in API v2 docs

Add a "real" example of a groupby statistics query with return
values in API v2 documentation.

The example is taken from an actual query run on a machine
running Devstack and Ceilometer.

Addresses bug # 1228465

Change-Id: I0414fdaf31ca365f0404c77504d4106de7c17f74
Merge "Add example with return values in API v2 docs"