Get Post Metadata

You need to update the `functions.php` file in WordPress and include the following code to allow metadata to be updated via the API:

function expose_metadata() {
      register_rest_field(
           'post',
           'post_meta_fields',
           array(
                'get_callback' => 'callback_read_post_meta',
                'update_callback' => 'callback_update_post_meta',
                'schema' => null,
           )
      );
}

function callback_read_post_meta( $object ) {
      $post_id = $object['id'];
      return get_post_meta( $post_id );
}

function callback_update_post_meta( $meta_value, $object, $field_name ) {
      $post_id = $object->ID;

      if (!is_array( $meta_value ) ) {
           return new WP_Error( 'rest_invalid_param', __( 'El valor debe ser un array de campos de metadatos.' ), array( 'status' => 400 ) );
      }

      foreach ( $meta_value as $meta_key => $meta_value ) {
           update_post_meta( $post_id, sanitize_key( $meta_key ), sanitize_text_field( $meta_value ) );
      }
      return true;
}

add_action( 'rest_api_init', 'expose_metadata' );