ETA for Reindex API

[dvisztempacct]

The get task API (e.g. GET _tasks/lOHn4ATPST6LfKCW6m539w:517820) should include an estimated time of completion where applicable.

Specifically when using the reindex API, I had to write myself a basic tool some time ago to estimate when the task would be completed using the total, created, updated, start_time_in_millis, and running_time_in_nanos properties. (Although I believe in cases where the reindex operation omits some documents, this may not even be sufficient!)

It would be great if this functionality was built into this API.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)

[DaveCTurner]

I don't think it makes sense to be able to compute an ETA for all tasks exposed by the tasks API; some of them will just be waiting for other things to happen and even the ones that are actively working won't necessarily have any well-defined and predictable endpoint. But reindex (and perhaps update-by-query and delete-by-query) are special and maybe it does make sense for them. I updated the title to reflect this.