Remove references to MetadataOutput from dev guide

This is now a much more 'internal' component, and there's no way to
register one directly on a `Player` instance.

Issue: androidx/media#258

#minor-release

PiperOrigin-RevId: 513849776
(cherry picked from commit abcb8061)

docs/ad-insertion.md

@@ -310,9 +310,9 @@ metadata.
 
 Often, server-side inserted ad streams contain timed events to notify the player
 about ad metadata. Please see [supported formats][] for information on what
-timed metadata formats are supported by ExoPlayer. Custom ads SDK `MediaSource`s
-can listen for timed metadata events from the player, e.g., via
-`ExoPlayer.addMetadataOutput`.
+timed metadata formats are supported by ExoPlayer. Custom ads SDK `MediaSource`
+implementations can listen for timed metadata events from the player via
+`Player.Listener.onMetadata`.
 
 [VAST]: https://www.iab.com/wp-content/uploads/2015/06/VASTv3_0.pdf
 [VMAP]: https://www.iab.com/guidelines/digital-video-multiple-ad-playlist-vmap-1-0-1/

docs/retrieving-metadata.md

@@ -22,10 +22,10 @@ public void onMediaMetadataChanged(MediaMetadata mediaMetadata) {
 {: .language-java}
 
 If an application needs access to specific [`Metadata.Entry`][] objects, then it
-should add a `MetadataOutput` (for dynamic metadata delivered during
-playback) to the player. Alternatively, if there is a need to look at static
-metadata, this can be accessed through the `TrackSelections#getFormat`. Both of
-these options are used to populate the `Player#getMediaMetadata`.
+should listen to `Player.Listener#onMetadata` (for dynamic metadata delivered
+during playback). Alternatively, if there is a need to look at static metadata,
+this can be accessed through the `TrackSelections#getFormat`.
+`Player#getMediaMetadata` is populated from both of these sources.
 
 ## Without playback ##