Add testcase for multiple discovery-driven sources

This test was intended to reproduce the reported issue around
resources discovered for one pollster bleeding into the resource
list passed to an unrelated pollster with the same interval.

However the observed behaviour seems correct.

Related-bug: #1335986
Change-Id: Ifb608246ad2ee2d858b4268ae2e8dc6faaff025c
Add cumulative and gauge to aggregator transformer

Currently, the aggregator transformer just sums up the volumes of all
aggregated samples.

This patch improves it so it returns just the latest volume for cumulative
meters and the average of volumes for gauge meters.

DocImpact: Aggregator transformer now works for gauge and cumulative as well
Change-Id: I172a7298dc063ff36b2a209df25d6650debb5a14
Closes-bug: #1342668
Merge "Add testcase for multiple discovery-driven sources"
Merge "Add cumulative and gauge to aggregator transformer"