Labels are consistently validated across services

* The combined size of a key/value pair cannot exceed 4096 bytes

Signed-off-by: Jess Valarezo <valarezo.jessica@gmail.com>

api/services/containers/v1/containers.proto

@@ -42,6 +42,8 @@ message Container {
 
 	// Labels provides an area to include arbitrary data on containers.
 	//
+	// The combined size of a key/value pair cannot exceed 4096 bytes.
+	//
 	// Note that to add a new value to this field, read the existing set and
 	// include the entire result in the update call.
 	map<string, string> labels  = 2;