The introductory paragraph on the developer page for the MediaState trait implies that the MediaState trait lets you provide Assistant with information about the media item your device is currently playing... The examples it gives suggest it includes way to provide a user readable title for the media being played, as well as a URI to the actual media stream and information about the viewer's current position within the stream so that playback could be moved to another device.
However, looking at the actual Device STATES section of the page, all I see is a schema for telling Assistant whether the device is currently playing / paused / stopped / fast forwarding / etc... Nothing for providing information about the media item being played.
Did I miss something or has Google simply not fully implemented (or not fully documented) this trait?
Apologies for any confusion created by the current documentation. The schemas presented in the trait reference are accurate in that
MediaStatecurrently only supports reporting the state of activity and playback controls in conjunction with the TransportControl trait.The intent is to provide support for more descriptive media items in the future, but that is not currently part of the API. This is an area where we could use feedback on what types of metadata would best suit your use case, so I would recommend filing a feature request on the public tracker.