Player metrics: ttff, user agent, autoplay flag

[pglowacky]

Time to first frame

This metric is used to measure the amount of time (in seconds) that a user must wait for the video to play after indicating that they'd like the video to play (either by pressing the play button or autoplay).

Must capture:

• Timestamp when user hits the play button (or autoplay is triggered)
• Timestamp when the viewer sees the first frame.

@ecmulli Can you fill in details about how this data should be captured so that we can query it?

Edited after clarification:
Must track:

1. ttff
2. user-agent
3. autoplay flag