per https://docs.google.com/document/d/1fFTVFxvsOfd9DdAYPGDPXfHH3o3Pa0Po4rOfr1TYM3o/edit?pli=1#heading=h.c59uqgtprkrp, we would like to add trace events to CorePageLoadMetrics to get OnLoad/FirstContentfulPaint/FirstMeaningfulPaint out of PLM, and replace what we have in loading metrics with output from PLM eventually.