In-Progress Resource Requests

Hi all,

In the last call we decided we'd gather concrete use cases and then
continue the discussion to decide the shape of the API for in-progress
resource requests. I've written a short comment [1] on what I've gathered
along with my thoughts on what the best pattern would be (TL;DR: using
PerformanceObserver). Let me know what your thoughts directly on the issue,
thanks!

[1] https://github.com/w3c/resource-timing/issues/137#issuecomment-455249030