Fix AppEngine variant of go_proto_library: also needs gRPC metadata dependency

This was forgotten in https://github.com/bazelbuild/bazel/commit/52035c1c2d0ad494fb4318a6c10a6a46749ea6a2 and manifested itself in test failures (see
http://b/111056578#comment5).

PiperOrigin-RevId: 210318372
diff --git a/src/test/java/com/google/devtools/build/lib/packages/util/MockProtoSupport.java b/src/test/java/com/google/devtools/build/lib/packages/util/MockProtoSupport.java
index b70f0a0..e8a9fc4 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/util/MockProtoSupport.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/util/MockProtoSupport.java
@@ -185,7 +185,8 @@
         "third_party/golang/grpc/metadata/BUILD",
         "package(default_visibility=['//visibility:public'])",
         "licenses(['notice'])",
-        "exports_files(['metadata'])");
+        "go_library(name = 'metadata',",
+        "           srcs = [ 'metadata.go' ])");
   }
 
   /**