Default unity of measurement

How can I set a default unity of measurements like meters or centimeters, to use for every product… instead of using the default “units” I need to create my own default. Already tried something like this, but I’ve got no luck…

add_action('save_post_product', 'set_default_unit');

function set_default_unit( $post_ID ) {

				update_post_meta(
					$post_id,
					'unit',
					'metros'
				);

}