Convert pair returned by first/last aggregators to number (#5566)

[kskalski]

No description provided.

[jihoonson]

@kskalski, thank you for the contribution!

I believe this PR would be useful for some use cases, but existing implementation is also useful as well. So, I think it would be better to add an option to First/LastAggregatorFactory to get only time/value or both. The default should be both as it does.

[kskalski]

Yes, I guess it would be a better fix to get the right value when processing is done. Actually First/LastAggregator already have methods for fetching just the value or timestamp, however I don't know the code-base enough to find out when/where they should be called.
I'm not sure where the values surfaced in the exception are actually produced, both *AggregatorFactory and Aggregator have methods, which return SerializablePair. My guess is that somewhere the code calls Aggregator::get instead of Aggregator::getDouble, which for First/Last Aggregator returns a pair. Unfortunately the exception I get is already past that phase, values are already produced and there is only symptomatic way to deal with it.

[jihoonson]

@kskalski sorry for the delayed review. I'm closing and reopening this PR to trigger Travis build. The link is not available now.

[jihoonson]

Hi @kskalski, please check my comment here: #5566 (comment). I think this is not a bug, but we need a better doc for this.

I'm closing this PR shortly.