Added `notUsed` as player operation metric for Tracking event attribu…

[fabiant7t]

Added notUsed to the list of valid Tracking event attributes.

3.14.1 Tracking Event Descriptions (Source: VAST 4.2 PDF)

Player Operation Metrics (for use in Linear and NonLinear Ads)

notUsed: This ad was not and will not be played (e.g. it was prefetched for a particular ad break but was not chosen for playback). This allows ad servers to reuse an ad earlier than otherwise would be possible due to budget/frequency capping. This is a terminal event; no other tracking events should be sent when this is used. Player support is optional and if implemented is provided on a best effort basis as it is not technically possible to fire this event for every unused ad (e.g. when the player itself is terminated before playback).