[KafkaIO] Report unknown backlog size when latest offset lags behind next offset

[sjvanrossum]

The periodic refresh of a partition's end offset may lag behind the consumption from that partition.

Instead of reporting no backlog when latestOffset < nextOffset the backlog should be reported as unknown (represented by -1).

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Reminder, please take a look at this pr: @kennknowles @liferoad

[kennknowles]

I actually wonder if this could have weird issues? I guess the presumption of the prior code is that we are "caught up" if the next offset has caught up to the latest offset. This could be temporarily incorrect, waiting on the next refresh, but that could be better than oscillating between unknown & known backlog numbers?

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Reminder, please take a look at this pr: @kennknowles @liferoad

[liferoad]

I actually wonder if this could have weird issues? I guess the presumption of the prior code is that we are "caught up" if the next offset has caught up to the latest offset. This could be temporarily incorrect, waiting on the next refresh, but that could be better than oscillating between unknown & known backlog numbers?

@sjvanrossum are we good to merge this PR?

[sjvanrossum]

@liferoad yes, this is good to merge. 👍