Adding page to reportWebVitals payload

[jackjdleslie]

Context

Collecting web vitals through default Next.js Analytics allows you to view metrics per page, however the custom reportWebVitals payload doesn't contain the data for page.

Solution

The way that it's done currently via Next.js Analytics is by reading window.__NEXT_DATA__.page, so we can use this same mechanism with reportWebVitals.

Implementation

Issue: #23457
Pull Request: #23458