message: Add missing Null VM MetadataType using directive.
diff: @@ -18,6 +18,7 @@ using FilterTrailersStatus = Http::FilterTrailersStatus;
  using FilterDataStatus = Http::FilterDataStatus;
  using GrpcStatus = Envoy::Grpc::Status::GrpcStatus;
  using MetricType = Envoy::Extensions::Common::Wasm::Context::MetricType;
  +using MetadataType = Envoy::Extensions::Common::Wasm::MetadataType;
  using StringView = absl::string_view;
  template <typename T> using Optional = absl::optional<T>;
  } // namespace Plugin
  