feat(dashviewer): Video performance instrumentation

[ConradJChan]

Introduces two new metric events:

• media_metric_buffer_fill which represents the initial buffering before playback can begin
• media_metric_end_playback which contains metrics around total buffering time, lag/watch ratio, etc...

media_metric_buffer_fill will be emitted as soon as it is measured.

media_metric_end_playback will be emitted in 3 scenarios:

1. when playback is finished (ended event is fired)
2. when the viewer is destroyed (when preview is closed)
3. when the page unloads

As part of this PR, only the DashViewer will emit these metrics.

TODO:

• fix unit tests
• add new unit tests

[boxcla]

Verified that @ConradJChan has signed the CLA. Thanks for the pull request!

[mickr]

Seems like this is more than a chore to me