Add 'extra' metadata Attribute Options in SearchWP

searchwp-customizations.php

<?php

// Add 'extra' metadata Attribute Options in SearchWP.
add_filter( 'searchwp\source\attribute\options', function( $options, $args ){
	if( $args['attribute'] !== 'meta' ){
		return $options;
	}

	$these_keys = [ 'my_custom_key' ];

	foreach( $these_keys as $this_key ){
		// Add this field if it's not added already.
		if( ! in_array(
			$this_key,
			array_map( function( $option ){ return $option->get_value(); }, $options )
		) ){

			// Each option must be a \SearchWP\Option.
			$options[] = new \SearchWP\Option( $this_key, 'Extra Metadata: ' . $this_key );
		}
	}

	return $options;
}, 10, 2 );