Make Query Monitor available for logged out users. Useful alternative when Query Monitor's own authentication cookie-based support for the same thing is not enough.

query-monitor-for-logged-out-users.php

<?php

/**
 * Makes Query Monitor output available to all users.
 *
 * This can, for example, be added to mu-plugins/query-monitor.php (and can
 * easily be disabled by changing 'init' to 'xinit', or commenting the whole
 * thing out, or adding a return statement, etc).
 *
 * Reasons you might use this instead of simply setting Query Monitor's own
 * authentication cookie include cases where you are debugging locally and
 * wish to remain logged in as an admin user in one tab, and/or are also using
 * one or more private windows (or container tabs) concurrently.
 */
add_action( 'init', function() {
    $user = wp_get_current_user();

    if ( is_object( $user ) ) {
        $user->add_cap( 'view_query_monitor' );
    }
} );