Allows to get aggregated measurements of metrics

This allows to get aggregated measurements of metrics
via a new kind of endpoint:

  /v1/resource/instance/server_group=foobar∧display_name=ex/ \
     metrics/cpu_util?aggregation=max

to aggregated the measurements of metrics that are instance with
attributes server_group set to foobar and display_name set to ex

Only '=' attribute operator and '∧ (LOGICAL AND)' for joining attributes
are implemented.

Change-Id: I476cd0eb02ea818707ec800244ed5c76e4cb9c75
Merge "Allows to get aggregated measurements of metrics"