Add flag to show whether intrasite navigation

[tunetheweb]

If the page is an intra-site navigation, then it likely has different performance characteristics to a cross site navigation which may have a higher TTFB and/or less objects cached.

This could be measured with new URL(document.referrer).origin === new URL(performance.getEntriesByType("navigation")[0].name;)

Where should this be exposed? In the TTFB attribution model?
Or as general additions to the Metric object so available for all metrics?

[tunetheweb]

Agreed to put in TTFB attribution model.