Add Metadata.toString that prints the contents of `entries`

entries are used in .equals(), so it's good to have them printed in toString() too (for test failures) and it makes logging easier too.

PiperOrigin-RevId: 263335503

library/core/src/main/java/com/google/android/exoplayer2/metadata/Metadata.java

@@ -122,6 +122,11 @@ public final class Metadata implements Parcelable {
     return Arrays.hashCode(entries);
   }
 
+  @Override
+  public String toString() {
+    return "entries=" + Arrays.toString(entries);
+  }
+
   // Parcelable implementation.
 
   @Override