Gist for my article Building a Gutenberg sidebar plugin Part 5: Outputting data https://vanaf1979.nl/building-a-gutenberg-sidebar-plugin-part-5-outputting-data/

metatags-add-meta-tags.php

<?php
public function add_metatags_to_head() : void {

    global $post;
    
    foreach( $this->metafields as $key => $value ) {

        if ( $key == 'title' ) {

            continue;

        }
        
        $value = \trim( \get_post_meta( $post->ID , $value , true ) );

        echo "\t<meta name=\"{$key}\" content=\"{$value}\" />\n";

    }

}