feat(browser): Report web vitals for bfcache restores - #23750
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`withoutBfcache` dropped every metric web-vitals reported after a
back/forward-cache restore. That was the right call while there was
nothing to attach them to: a restore reuses the frozen document, so the
values would have landed on the span the page had before it was frozen.
Now that a restore gets its own navigation span, they have a correct
parent, so the drop becomes an option rather than a hard rule:
webVitals: { bfcache: true }
Off by default. A restore is near-instant, so its vitals are a different
population from page load vitals, and the earlier concern about skewing
aggregates still applies to anyone who has not decided how to treat them.
`browser.navigation.type: bfcache` makes them separable once enabled.
Reporting per navigation rather than per page load is now what the
tracker flag means, since bfcache restores need it for the same reason
soft navigations do. `reportAllChanges` is switched off for either, since
the per-navigation path relies on each reported value already being final
for its navigation.
Verified end to end in Chrome 152: a restore emits LCP and CLS parented
to the bfcache navigation span on the restore's own trace, and a
bfcache-ineligible back navigation still falls back to a page load.
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Reports web vitals for back/forward cache restores, which we dropped outright before.
Now that a restore gets its own navigation span in #23748, the vitals have a correct parent, so the hardcoded
withoutBfcachedrop becomeswebVitals: { bfcache: true }. Off by default, since a restore is a different population from a page load.Stacked on #23748.