I will qualify my contribution firstly by stating that I am not a technologist; however, I think this generalizes to task complexity and performance no matter the specialty.
Complexity is a metric itself. However, it is qualitative and subjective. So it sounds like you are looking for a quantitatively measured condition that is always present with complexity, so that if and how much the condition is present you can assume reasonably that the requirement was therefore complex.
Time, by itself, does not cut it. Indeed, a complex task will take time. But there are factors of time that will skew the results and provide false positives: Time is relative. What is 'a long time?' If you measure duration, other events can interfere causing the duration to expand. If you measure work hours, then you have to somehow normalize it depending on the skill level of the resource(s) used or the typical op tempo one resource has over another, i.e., some people work quickly, others slowly.
Finally, some easy tasks simply take a long time.
I do not know if you will find a single quantitative metric, but rather the presence of multiple conditions to indicate complexity, e.g., time, number of defects, re-work, need for consultation and extensive peer reviews, and finally the subjective opinion of those who worked it.
After all, complexity is subjective. You need to conclude complexity after human analysis of the total picture. A quick, single measurement will not cut it.