How can I display image metadata?

Because the credit data is saved in the attachment’s post meta, not the main post:

$credit = get_post_meta( $post->ID /* Wrong ID! */, 'credit', true );

Instead you need to catch the ID of the inserted image:

function attach_image_credit( $images ) {
    $return = $images[0];

    // Get the image ID from the unique class added by insert to editor: "wp-image-ID"
    if ( preg_match( '/wp-image-([0-9]+)/', $return, $match ) ) {
        if ( $credit = get_post_meta( $match[1] /* Captured image ID */, 'credit', true ) )
            $return .= $credit;
    }

    return $return;
}