[webrtc-stats] Need metrics for capture glitches (#678)

henbos has just created a new issue for https://github.com/w3c/webrtc-stats:

== Need metrics for capture glitches ==
Similar to #676 but regarding capture rather than playout. We can add these to RTCAudioSourceStats.

If a media source or audio capture pipeline does not consume samples provided from a capture device on time they are dropped.
- RTCAudioSourceStats.totalSamplesDuration: already exists.
- RTCAudioSourceStats.droppedSamplesDuration: new metric. Incremented every time we drop samples in the capture pipeline.


Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/678 using your GitHub account


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