Callback duration metric

[benbierens]

All right... this doesn't currently work and I need some help.

Here's the goal: As a chronos user, I want to be able to measure callbacks durations (when using debug builds), so that I can be informed about long callbacks that will cause my app to block/not respond.

I've added a compile-flag and some code to asyncloop.nim to try and get this going, only to realize that chronos doesn't use nim-metrics. (And can't because nim-metrics uses chronos?) So I'm writing this message to ask help from you, experienced chronos developers! Do you see a use for a feature of this sort? How can this best be accomplished?