Generates metadata that is needed specifically for this editor.
Will only be called by \Drupal\edit\MetadataGeneratorInterface::generate() when the passed in field instance & item values will use this editor.
\Drupal\field\Plugin\Core\Entity\FieldInstance $instance: The field instance of the field being edited.
array $items: The field's item values.
array A keyed array with metadata. Each key should be prefixed with the plugin ID of the editor.
public function getMetadata(FieldInstance $instance, array $items);