How to get author meta into post endpoint in api v2

This should work:

function wpse_20160421_get_author_meta($object, $field_name, $request) {

    $user_data = get_userdata($object['author']); // get user data from author ID.

    $array_data = (array)($user_data->data); // object to array conversion.

    $array_data['first_name'] = get_user_meta($object['author'], 'first_name', true);
    $array_data['last_name']  = get_user_meta($object['author'], 'last_name', true);

    // prevent user enumeration.
    unset($array_data['user_login']);
    unset($array_data['user_pass']);
    unset($array_data['user_activation_key']);

    return array_filter($array_data);

}

function wpse_20160421_register_author_meta_rest_field() {

    register_rest_field('post', 'author_meta', array(
        'get_callback'    => 'wpse_20160421_get_author_meta',
        'update_callback' => null,
        'schema'          => null,
    ));

}
add_action('rest_api_init', 'wpse_20160421_register_author_meta_rest_field');