WordPress at a glance
function is not described

metadata_exists() WP 3.3.0

Determine if a meta key is set for a given object

Hooks in function
Return

true/false. True of the key is set, false if not.

Usage

metadata_exists( $meta_type, $object_id, $meta_key );
$meta_type(string) (required)
Type of object metadata is for (e.g., comment, post, term, or user).
$object_id(int) (required)
ID of the object metadata is for
$meta_key(string) (required)
Metadata key.

Code of metadata exists: wp-includes/meta.php VER 5.1.1

<?php
function metadata_exists( $meta_type, $object_id, $meta_key ) {
	if ( ! $meta_type || ! is_numeric( $object_id ) ) {
		return false;
	}

	$object_id = absint( $object_id );
	if ( ! $object_id ) {
		return false;
	}

	/** This filter is documented in wp-includes/meta.php */
	$check = apply_filters( "get_{$meta_type}_metadata", null, $object_id, $meta_key, true );
	if ( null !== $check ) {
		return (bool) $check;
	}

	$meta_cache = wp_cache_get( $object_id, $meta_type . '_meta' );

	if ( ! $meta_cache ) {
		$meta_cache = update_meta_cache( $meta_type, array( $object_id ) );
		$meta_cache = $meta_cache[ $object_id ];
	}

	if ( isset( $meta_cache[ $meta_key ] ) ) {
		return true;
	}

	return false;
}