`final` v. `not final` labels were a bit unclear at first about web-vitals-extension HOT 3 CLOSED

@philipwalton for some alignment with the library...

How would we feel about the extension using (computed) and (still computing)?

I admit that if you haven't been a reader of some of Phil's recent docs, the notion of final/non-final isn't going to be all that clear and can imagine others being confused about what these terms mean.

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#32 attempts to address this.

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This isn't really an answer to your question, but one thing I've been thinking about w.r.t. the isFinal flag, is how it'll play out when we add bfcache to Chrome.

In those cases, I plan to reset all the metric values, and re-run the callbacks, but I'm not sure if that'll be unexpected in cases where a result was previously marked as "final".

My initial thinking is I'll add some sort of sessionID property to the metric object, which will change in the event that a page is taken out of the bfcache (or if we decide to add session boundaries on visibilitychange for long-lived pages). This will allow the metric value to be disambiguated by analytics tools from the previously sent value for the same page load, and I could say in the docs that isFinal only applies to the value from the current "sessionID".

Not sure if this will affect the extension, but I suppose it could if you're removing any listeners once the value finalizes.

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