fix-775 fix sdk promote missing annotations (kubeflow#777)

* fix-775

* add metadata directly

* fix metadata tyep error

python/kfserving/kfserving/api/kf_serving_client.py

@@ -289,12 +289,18 @@ def promote(self, name, namespace=None, watch=False, timeout_seconds=600): # pyl
         except KeyError:
             raise RuntimeError("Cannot promote a InferenceService that has no Canary Spec.")
 
+        try:
+            annotations = current_isvc['metadata']['annotations']
+        except KeyError:
+            annotations = dict()
+
         inferenceservice = V1alpha2InferenceService(
             api_version=api_version,
             kind=constants.KFSERVING_KIND,
             metadata=client.V1ObjectMeta(
                 name=name,
-                namespace=namespace),
+                namespace=namespace,
+                annotations=annotations),
             spec=V1alpha2InferenceServiceSpec(
                 default=current_canary_spec,
                 canary=None,