Adding resource discovery time to ResourceTiming API structure

[giacomozecchini]

The structure of the ResourceTiming API does not include details about the discovery time of a specific resource.

One potential use case for this new attribute is to analyze whether the Largest Contentful Paint element is discovered late during the rendering process.

[clelland]

We talked about this in the last WG meeting, and it seems there would be some work involved in specifying exactly when "discovery" occurs (especially in the preload scanner, as that's likely not well-defined anywhere). I think that we'd also like to know what kind of support this has; whether there are enough real world use cases.

[nicjansma]

Notes from the April 13th 2023 W3C WebPerf Working Group meeting: https://docs.google.com/document/d/1fIgfd7ONz-sES55qRFHQI_eQUnynnyX2Asrs-cwAQ_Q/edit#heading=h.74m0x7yxo8eu