Add to telemetry how big page json & page bundle sizes

[KyleAMathews]

probably just send descriptive statistics summary e.g. mean, median, max, min, std dev, & some measure of skew.

We want to warn against sending too much code/data on pages & it'd be good to keep track of how people are doing at the moment.

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[sidharthachatterjee]

Fixed in #20146