Fix a typo in metrics.proto. No code is using double_value yet and hence no...

Fix a typo in metrics.proto. No code is using double_value yet and hence no other code changes are needed

src/proto/grpc/testing/metrics.proto

-// Copyright 2015, Google Inc.
+// Copyright 2015-2016, Google Inc.
 // All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
@@ -42,7 +42,7 @@ message GaugeResponse {
   string name = 1;
   oneof value {
     int64 long_value = 2;
-    double double_vale = 3;
+    double double_value = 3;
     string string_value = 4;
   }
 }