[Feature request]: Collapse all threads onto "meta" performance profile

While having the ability to look at the performance profiles for individual threads separately is great as there's no information loss that way, doing what the original FlameGraph did in collapsing all threads into the same, single flamegraph also has its own advantages.

This is especially true when you want more general information on "which function consumed the most CPU time" for example, irrespective of how this CPU time was distributed across threads.

It is my understanding that such feature is not supported at the moment. Assuming I am correct in this, I wonder whether adding such feature would be possible and if it would be particularly difficult.

Combining them while maintaining the "Time Order" might be more difficult, but perhaps we could limit this to the "Left Heavy" and "Sandwich" views.

Perhaps, some "whole system" option could be added to the the profile/thread selector.