Feat: Time on task

[bmtcril]

A number of v1 reports require a concept of "time spent" / "how long". Due to the nature of the web these questions are always complicated, such as the "sandwich problem" (how can we tell if the user gets up to make a sandwich?). This task is to research the current state of this problem in web analytics and suggest a solution that we can use to generate these metrics at a reasonable level of certainty.

AC:

• A document is created in this repository describing how we will measure this, which we can present to users and also use as a specification for our timing-based reports

[Ian2012]

@bmtcril are there any tracking logs events that can provide a baseline? or should we propose new tracking log events to support this metric(s)?

[bmtcril]

@Ian2012 I think it's going to be different from type to type. We have pretty good ideas on video watches, for instance, and for "how long did it take to complete this course", but things like time to answer a problem may require some other kind of eventing. We may not want to support some of that if the events are too invasive (tracking mouse / keyboard activity or what's visible on screen).