[webrtc-stats] Add counters for the subset of concealedSamples that are not silent (#409)

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

== Add counters for the subset of concealedSamples that are not silent ==
There is a qualitative difference between concealed samples that were produced as silence and concealed samples that were produced as non-silence. Having counters for this is important in order to measure audio quality.

Proposal:
- audibleConcealedSamples: The total number of concealed samples inserted that are non-silent. This is a subset of concealedSamples.

Products that are relying on Chrome's non-standard googSpeechExpandRate could use this metric instead. This unblocks migration efforts on such products.

Please view or discuss this issue at https://github.com/w3c/webrtc-stats/issues/409 using your GitHub account