Add label attribute for audio and captions as optional [3.2.4. Presence of Attributes and Elements]

[itaykinnrot]

Hi, i would like to suggest that we'll add label attribute for audio and captions.
When playing a stream with multiple audio tracks, currently the only way to identify the different tracks is using the lang attribute of AdaptationSet. We have customers who would like to customize the label that is displayed to the end user for selecting the audio track. Also, multiple audio tracks are sometimes used for uses cases which are not multiple languages, for example - one audio track with background music and one without it.
While it is possible to pass this info out-of-band, we feel it makes most sense to pass it in-band in the MPD. Other protocols have support for this - in HLS you can set the NAME attribute of EXT-X-MEDIA, and in MSS there's a Name attribute on StreamIndex

[giladna]

+1

[itaykinnrot]

Following our conversation adding shaka-player merged pull and Exo player suggested pull:
shaka-project/shaka-player#831
shaka-project/shaka-player#811
google/ExoPlayer#2904

Thanks! @wilaw