extend Sending Duration for Flows

[robin2610]

	case duration, hardAge, idleAge:
		// ignore those fields.
		continue
	}

Currently duration is not sent as part of DumpFlowsWithFlowArgs.

I have a usecase where i have to check whether a flow is newly created or was added before some time interval.

Is there any reason we are not exposing duration as part of DumpFlows? Can we add it , happy to contribute.