add Accumulated GC Duration to kettle config

The agent now supports aggregating metrics for a single metric definition, so we need to make sure we add this to the standalone config XSL for the kettle build once the agent that supports this is released and kettle is configured to use that agent release:

<metric-dmr name="Accumulated GC Duration" interval="1" timeUnits="minutes" path="/core-service=platform-mbean/type=garbage-collector/name=*" attribute="collection-time" />