*: support watch with filters

Now user can filter events with types. The API is also extensible.
It might make sense for the proxy to filter out events based on
more expensive/customized filter.

Documentation/dev-guide/apispec/swagger/rpc.swagger.json

@@ -181,6 +181,15 @@
       ],
       "default": "KEY"
     },
+    "WatchCreateRequestFilterType": {
+      "type": "string",
+      "enum": [
+        "NOPUT",
+        "NODELETE"
+      ],
+      "default": "NOPUT",
+      "description": "- NOPUT: filter out put event.\n - NODELETE: filter out delete event."
+    },
     "authpbPermission": {
       "type": "object",
       "properties": {
@@ -1171,6 +1180,13 @@
     "etcdserverpbWatchCreateRequest": {
       "type": "object",
       "properties": {
+        "filters": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/WatchCreateRequestFilterType"
+          },
+          "description": "filters filter the events at server side before it sends back to the watcher."
+        },
         "key": {
           "type": "string",
           "format": "byte",