outbound: Add 'port' to resource metadata (#227)

The gateway API supports port references, especially when referencing a
Service. This metadata should be included with the rest of these
resource references.

Author: olix0r

go/meta/meta.pb.go

@@ -26,4 +26,5 @@ message Resource {
   string name = 3;
   string namespace = 4;
   string section = 5;
+  uint32 port = 6;
 }