Getting all meta data
This example demonstrates leaving the $key argument blank, in order to retrieve all meta data for the given user (in this example, user_id = 9):

<?php
  $all_meta_for_user = get_user_meta( 9 );
  print_r( $all_meta_for_user );
?>

Results:

Array ( [first_name] => Array ( [0] => Tom ) [last_name] => Array ( [0] => Auger)
[nickname] => Array ( [0] => tomauger ) [description] => etc.... )

Note: In order to access the data in this example you need to dereference the array that is returned for each key, like so:

$last_name = $all_meta_for_user['last_name'][0];

To avoid this, you may want to run a simple array_map() on the results of get_user_meta() in order to take only the first index of each result (thus emulating what the $single argument does when $key is provided:

  $all_meta_for_user = array_map( function( $a ){ return $a[0]; }, get_user_meta( $user_id ) );
  print_r( $all_meta_for_user );

Results:

Array ( [first_name] => Tom [last_name] => Auger [nickname] => tomauger [description] => etc.... )

Additionally, if you want to return ALL meta for a specific user and filter out empty values, you can run array_filter() on the results of the array_map() above:

// Get all user meta data for $user_id
$meta = get_user_meta( $user_id );

// Filter out empty meta data
$meta = array_filter( array_map( function( $a ) {
	return $a[0];
}, $meta ) );