Fine Performance Metrics and Spans

While it’s trivial to measure the end-to-end runtime of a Dask workload, the next logical step - breaking down this time to understand if it could be faster - has historically been a much more arduous task that required a lot of intuition and legwork, for novice and expert users alike. We wanted to change that. There are a few questions that someone who wants to optimize the performance of their Dask workflow typically asks: How much time am I losing to GIL contention ?