Meta Query Key for String [duplicate]

Question

My posts have meta keys _vote_user . user_id() and value of (int) 1, 2 or 3.

I would like to return all posts with a meta KEY containing the string '_vote_user', i.e.: return all posts with meta vote for the current round but by any user.

$meta_query = [
    [
        'key'       => '_vote_user_',
        'value'     => $atts['round'],
        'compare'   => 'LIKE'
    ]
];

As I understand it though, with the above code I'm querying the value for a LIKE value, not the key. How can I query the key? Maybe this illustrates better what I'm trying to achieve:

$meta_query = [
    [
        'key'       => '_vote_user_%', // where '%' is any integer
        'value'     => $atts['round'],
        'compare'   => 'EXISTS'
    ]
];

EDIT

What if I were to first identify the IDs of the judges and query just those possible keys? So if my judges IDs were 2, 3 and 5 would the only way to query that be:

$meta_query = [ 
        'relation' => 'OR',     
        [
            'key'       => '_vote_user_2',
            'value'     => $atts['round'],
        ],[
            'key'       => '_vote_user_3',
            'value'     => $atts['round'],
        ],[
            'key'       => '_vote_user_5',
            'value'     => $atts['round'],
        ]
    ];

It doesn't seem to be possible to submit an array of keys like you can for values.