@Nonnull
    public static Optional<MetadataMap> getForPlayer(@Nonnull Player player) {
        return players().get(player);
    }
Gets a {@link MetadataMap} for the given object, if one already exists and has
been cached in this registry.

A map will only be returned if the object is an instance of
{@link Player}, {@link UUID}, {@link Entity}, {@link Block} or {@link World}.

@param obj the object
@return a metadata map
helper/src/main/java/me/lucko/helper/metadata/Metadata.java