Better identification of tasks in the UI

[romain-intel]

Summary

Currently, when wide foreaches (or worse, nested ones) are running, it is hard to tell which task corresponds to which "position in the foreach tree". For example, if you are doing a foreach on countries (let's say 3 countries) and inside you are doing a foreach on movie titles within each country (let's say 100 movies), your step per_title_compute for example would have 300 tasks and it is very hard to know, as they are running within the UI, which one is which. Identifying them and showing the country/title the task is processing would be very helpful. Even showing something like indices may help.

Motivation

This gives better visibility into what is executing.

Describe alternatives you've considered

One alternative is to print out the position of the task in the log at the beginning. This does give that information (so is useful) but doesn't make it highly visible in the UI and also does not make it filter-able. For example, if the user is looking for the execution of a particular country/title pair, they would have to click through 300 tasks to figure out which one they want to look at.