I'm wondering how services like fullstory or datadog's RUM actually work? My first thought was that they somehow stored the contents of the page and continually tracked events, but I'm not sure how viable that is? Looking at the request payloads that get sent with datadog's js sdk, I see big json blobs with some metrics (first contentful paint, etc), and some other data that's obfuscated. I'm just curious and I'm sure someone must know how these things work.