Key MUST be in an incomplete state (i.e. including a kind but not an ID
or name in its final pathElement).
This method will execute a service request.
Example:
$key = $datastore->key('Person');
$keyWithAllocatedId = $datastore->allocateId($key);
See also:
https://cloud.google.com/datastore/reference/rest/v1/projects/allocateIds allocateIds