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