How to display the 'key' as well as 'value' in custom fields

I'm using the following to display custom fields on the front end however, it only shows the 'value' How can I get it to also show the text contained in the 'key'?

<?php
// Display Custom Field Value
echo "<ul>";
echo "<li>".get_post_meta( $post->ID, 'Portrait', true )."</li>"; 
echo "<li>".get_post_meta( $post->ID, 'January', true )."</li>";
echo "<li>".get_post_meta( $post->ID, '28954', true )."</li>";
echo "</ul>";
?>