/**
* Create a single Key instance
*
* @see https://cloud.google.com/datastore/reference/rest/v1/Key Key
* @see https://cloud.google.com/datastore/reference/rest/v1/Key#PathElement PathElement
*
* @param string $kind The kind.
* @param string|int $identifier [optional] The ID or name.
* @param array $options [optional] {
* Configuration Options
*
* @type string $identifierType If omitted, type will be determined
* internally. In cases where any ambiguity can be expected (i.e.
* if you want to create keys with `name` but your values may
* pass PHP's `is_numeric()` check), this value may be
* explicitly set using `Key::TYPE_ID` or `Key::TYPE_NAME`.
* }
* @return Key
*/
public function key($kind, $identifier = null, array $options = [])
{
$options += ['namespaceId' => $this->namespaceId];
$key = new Key($this->projectId, $options);
$key->pathElement($kind, $identifier, $options);
return $key;
}