Ensure that Web Vitals are aggregated based on frame and sub frame

[ankur22]

This follows on from the POC #657.

At the moment, metrics are aggregated for all the pages that are navigated to. This is likely to be ok for an initial release, but eventually we may want to also aggregate based on the separate frames and sub frames from each page.

There's also a limitation with the Web Vitals JS library, which is that it doesn't work with sub frames. If we were to inject the JS library into sub frames too, how will we want to display the metrics? Can we simply aggregate the results from the root frame and its sub frames?