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