Currently, AMP experiments are controlled by binary config and cookie based user opt-in.

Here I propose an extra page level control using HTML meta data:

<meta name="amp-experiments-opt-in" content="experiment-a,experiment-b">

The priority of the 3 control are:
cookie (user opt-in/out) > meta (page level) > binary config (global)

This new mechanism has 2 major usages:

• For publishers to opt-in new features. For example, with this new mechanism, we don't need to globally launch feature like custom-ad to allow more testing. Individual publishers can opt-in on their own will.
• For amp-inabox to control experiment at serving time. Ad server can dynamically turn on/off experiment per ad request for AMP creatives served on non-AMP page. (A little bit tricky for AMP creatives served on AMP pages, as they share the same experiment flags with runtime. To not complicate things, we can ignore this meta settings, just like how we remove all the header scripts and CSS boilerplate.)

@cramforce @michaelkleber @keithwrightbos thoughts?