metric: Thread availability

[paulirish]

Capturing how available the main thread is. This is a proxy metric to for potential input latency, as we cannot predict when input will hit the browser.

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Update: See details at Timestamp: Main thread ready for interaction in #27

Scoring

S-curve handled by Ben's math. Basic expectations:

• Goal: 90% percentile main thread task duration ~ under 300ms
• Goal: Maximum main thread task duration is < 400ms
• 100/100 is 90% percentile figure is <12ms
• 0/100 is 90% percentile figure is <1000ms.
• TBD: Determine curve.