add count of tasks scheduled to time macro

[IanButterworth]

$ julia -t6 -q
julia> x = zeros(100);

julia> @time for i in x
           i
       end
  0.007487 seconds (4.00 k allocations: 204.148 KiB, 98.58% compilation time: 41% of which was recompilation)

julia> @time Threads.@threads for i in x
           i
       end
  0.046984 seconds (43.01 k allocations: 2.156 MiB, 7 tasks scheduled, 156.00% compilation time: 6% of which was recompilation)

julia> @time for i in x
           Threads.@spawn i
       end
  0.010576 seconds (5.74 k allocations: 295.383 KiB, 100 tasks scheduled, 96.17% compilation time: 60% of which was recompilation)

Might be too noisy for @time as tasks scheduled in itself isn't a bad thing to flag.. task switches instead?

[jpsamaroo]

Maybe @time could gain a flag to enable extra features like this? I personally would use both tasks scheduled and task switches when timing Dagger code, but that may be too noisy in general.

[IanButterworth]

Talking through this at juliacon the idea came up of instead showing if >10% of time was spent waiting on the scheduler.
Scheduling tasks itself isn't a bad thing, nor something that the user might want to minimize, so it's not a good fit compared to other metrics.

Reporting if >10% time was spent waiting on the scheduler seems like a generally suboptimal thing.