[Feature Request] Add task running progress (% complete) to the task management API

[kunisen]

It would be great if we can see the percentage of task completion.

For example, GET _tasks/xxx:nnn?human shows

{
  "completed" : false,
  "task" : {
    "node" : "YJJWO3lMT0ir9ORIlRkKZw",
    "id" : 17992692,
    "type" : "transport",
    "action" : "indices:data/write/reindex",
    "status" : {
      "total" : 1384254,
      "updated" : 543820,
      "created" : 5180,
...

It would be great if I can see the (updated+created)/total as %.

FYI, I found some APIs are already having that %.
Much appreciated if we could add this % to task management API as well.

#10805
#11393
#15975

[elasticmachine]

Pinging @elastic/es-core-features

[Leaf-Lin]

Another useful place to see would be under GET _cat/tasks API as an additional column, %completion. This might not be available for all tasks, but would be useful for some.

And from there, one can estimate "time till completion = running_time / %completion * ( 1-%completion)"

[martijnvg]

In order to have this kind information in cat task api, there should be some infrastructure in the task framework to easily retrieve a task's progress for tasks that support this. Otherwise this can't be implemented consistently.