add_filter( 'the_content', 'my_the_content_filter', 0 );
function my_the_content_filter( $content ) {
    if (is_single()) {
        global $post;
        $meta=get_post_meta($post->ID, 'meta', true);
        ob_start();
        ?>
            <div class="row">
              ... and the rest of the html
              ... and field you want to add to content
        <?php
        $content .= ob_get_clean();
    }
    return $content;
}