[FLINK-18656][network,metrics] Fix startDelay metric for unaligned checkpoints

[pnowojski]

Before this fix, startDelay metric was always set to zero for unaligned checkpoints.

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[rkhachatryan]

I think the change is correct but not enough: when registering a metric, AlternatingCheckpointBarrierHandler is used, not CheckpointBarrierUnaligner.
So we also need to override AlternatingCheckpointBarrierHandler.getCheckpointStartDelayNanos to delegate to activeHandler.

[pnowojski]

Yes, good catch @rkhachatryan . Can you take a look at the fixup commit?

[rkhachatryan]

Thanks, @pnowojski.
LGTM.