Provide more detailed metrics why unaligned checkpoint is taking long time

With unaligned checkpoint there can happen situation as in the attached screenshot.

Task reports long end to end checkpoint time (~2h50min), ~0s sync time, ~2h50min async time, ~0s start delay. It means that task received first checkpoint barrier from one of the channels very quickly (~0s), sync part was quick, but we do not know why async part was taking so long. It could be because of three things:

1. long operator state IO writes
2. long spilling of in-flight data
3. long time to receive the final checkpoint barrier from the last lagging channel

First and second are probably indistinguishable and the difference between them doesn't matter much for analyzing. However the last one is quite different. It might be independent of the IO, and we are missing this information.

Maybe we could report it as "alignment duration" and while we are at it, we could also report amount of spilled in-flight data for unaligned checkpoints as "alignment buffered"?

Ideally we should report it as new metrics, but that leaves a question how to display it in the UI, with limited space available. Maybe it could be reported as:

Where the values in the parenthesis would come from unaligned checkpoints.