You cannot print an array directly. The most basic return value you could use is var_export( $meta_values, TRUE );
.
This will create a text representation of the value.
Probably better is a little walker like
$output="";
foreach( $meta_values as $key => $value )
$output .= "$key = " . esc_html( end( $value ) ) . '<br />';
return $output;