global function to apply filter to custom field

I have no experience with the get_{$meta_type}_metadata filter either, hence all I can offer is a further approach to a possible workaround:

Iff you don’t need to satisfy third parties using get_post_meta, but only want to use the filter in your own code, you could write a custom wrapper for get_post_meta, which in turn will accept a filter:

function wpse94639_get_post_meta( $post_id, $key = '', $single = false ) {
    $metadata = get_post_meta( $post_id, $key, $single );
    return apply_filters( 'wpse94639', $metadata );
}

Now you could use add_filter( 'wpse94639', 'my_add_a_class_function' );.

This is only a viable solution, iff you don’t need to filter results when some other theme/plugin is using the original get_post_meta, obviously.