Add meta data field to a comment.

Signature

add_comment_meta( $comment_id, $meta_key, $meta_value, $unique = false )
comment_id
 (int) Comment ID.
meta_key
 (string) Metadata name.
meta_value
 (mixed) Metadata value.
unique
 (boolean) Optional, default is false. Whether the same key should not be added.
Default: false

Return

(int|boolean) Meta ID on success, false on failure.

Source

function add_comment_meta($comment_id, $meta_key, $meta_value, $unique = false) {
	$added = add_metadata( 'comment', $comment_id, $meta_key, $meta_value, $unique );
	if ( $added ) {
		wp_cache_set( 'last_changed', microtime(), 'comment' );
	}
	return $added;
}
WP Trac GitHub

Link here