Add speed index to Loading Speed

[paulirish]

A little curious that it's not included, especially since all the data is collected over in the bigquery repo.

Would it be trivial to add it to the UI?

[rviscomi]

Yeah there's a lot in BQ that we don't surface in the UI. It'd be pretty easy to add but the only question is if it's a valuable metric. In what ways would speed index be useful to you?

FWIW @igrigorik has expressed an interest in deprecating the existing Loading Speed report in favor of the field-based data in HA's CrUX report. As an extension of that, I've been considering revamping the UI to reflect the metrics included in the annual Almanac report.