Sanitize callback that treats data as HTML.

sanitize-as-html.php

<?php

function x_custom_metadata_sanitize_as_html( $field_slug, $field, $object_type, $object_id, $value ) {
	if ( is_array( $value ) )
		$value = array_map( 'wp_filter_post_kses', $value );
	else
		$value = wp_filter_post_kses( $value );

	return $value;
}