Implements hook_entity_operation_alter().
function entity_test_entity_operation_alter(array &$operations, EntityInterface $entity) {
$uri = $entity
->uri();
$operations['test_operation'] = array(
'title' => format_string('Test Operation: @label', array(
'@label' => $entity
->label(),
)),
'href' => $uri['path'] . '/test_operation',
'weight' => 50,
);
}