src/Metable/Hooks.php
public function setAttribute()
    {
        return function ($next, $value, $args) {
            $key = $args->get('key');

            if (!$this->hasColumn($key) && $this->allowsMeta($key) && !$this->hasSetMutator($key)) {
                return $this->setMeta($key, $value);
            }

            return $next($value, $args);
        };
    }
public function setAttribute()
    {
        return function ($next, $value, $args) {
            $key = $args->get('key');

            if (!$this->hasColumn($key) && $this->allowsMeta($key) && !$this->hasSetMutator($key)) {
                return $this->setMeta($key, $value);
            }

            return $next($value, $args);
        };
    }
Register hook on setAttribute method.

@return \Closure