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
);