Following discussion here.

examiner.js is a js tool that's served at https://cdn.ampproject.org/examiner.js and executed when user turn on #development=1 on an AMP page.

It prints extra debug info to developers, in addition to validator.js, which prints format rules violations.

A main use case can be to leverage browser API PerformanceObserver to monitor what 3P frames is exhausting resources and slowing things down.

@dvoytenko