amphtml: Support for time engaged since last timer trigger

What’s the issue?

Hi @rudygalfi and team, we’re looking to add engaged time support to the Parse.ly analytics tracker. Following #1296 and examples I’m seeing in the code, it seems that the recommended approach is to implement a timer trigger that uses a special request which appends ${totalEngagedTime}.

Parse.ly’s JavaScript tracker has an engaged time implementation that sends a “heartbeat” pixel every 10.5s which includes the total time engaged since page load and the amount of time engaged since the last heartbeat pixel. This greatly simplifies the server-side logic we have to do by allowing us to sum these increments.

In reviewing the analytics spec and available variable substitutions, I’m a stumped on how to reproduce “time engaged since last timer trigger” without a tricky server-side solution that tries to compute the delta between totalTimeEngaged on chronological pixels. Would you and the team have any suggestions on how to proceed?