How do I convert "COUNT(*) AS num" into the D7 API syntax?

How do I convert "COUNT(*) AS num" into the D7 API syntax. Following is what I have so far, but I'm not sure about the way I have added the COUNT expression:

    // D6 version of query:
//     db_query(
//       'INSERT INTO {statspro_term} (tid, ncount)
//         SELECT tid, COUNT(*) AS num
//         FROM {term_node}
//         GROUP BY tid'
//     );

    // D7 version of query?
    $query = db_select('taxonomy_term_node')
    ->fields(array('tid'))
    ->groupBy('tid');
    $query->addExpression("COUNT(*) as num");
    
    db_insert('statspro_term')
    ->from($query)
    ->execute();

Thanks!