refactor: added requestStart to the TTFB formula

[Zizzamia]

Super excited to see Time to First Byte joining the WebVitals family 🌲🚀🌕

I was wondering why TTFB is based only as navigationEntry.responseStart instead of being the combination of navigationEntry.responseStart - navigationEntry.requestStart. Doing this subtraction will imply not including browser set up, redirects, or tcp and ssl negotiation in the final result.

I originally learn about this way of calculating TTFB by the Assessing Loading Performance in Real Life with Navigation and Resource Timing article.

I made a tiny PR with this change in case you and the team believe navigationEntry.responseStart - navigationEntry.requestStart could work better to guide developer with this tool.

Thank you @philipwalton 🙌

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[philipwalton]

Thanks for the suggestion, but this library aims to track these metrics in a way that matches (as closely as possible) how they're tracked by Chrome and exposed in tools like the Chrome User Experience Report.

In the Chrome User Experience Report, TTFB measures the time between the navigation entry's responseStart property and the page's time origin (i.e. startTime).

To reduce confusion, though, I've submitted #12 which adds an example to the documentation clarifying this and also showing how you could use the getTTFB() function to measure what you've proposed here.