vendor: add "go-grpc-middleware/util/backoffutils"

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

vendor/github.com/grpc-ecosystem/go-grpc-middleware/wrappers.go

@@ -0,0 +1,29 @@
+// Copyright 2016 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+package grpc_middleware
+
+import (
+	"golang.org/x/net/context"
+	"google.golang.org/grpc"
+)
+
+// WrappedServerStream is a thin wrapper around grpc.ServerStream that allows modifying context.
+type WrappedServerStream struct {
+	grpc.ServerStream
+	// WrappedContext is the wrapper's own Context. You can assign it.
+	WrappedContext context.Context
+}
+
+// Context returns the wrapper's WrappedContext, overwriting the nested grpc.ServerStream.Context()
+func (w *WrappedServerStream) Context() context.Context {
+	return w.WrappedContext
+}
+
+// WrapServerStream returns a ServerStream that has the ability to overwrite context.
+func WrapServerStream(stream grpc.ServerStream) *WrappedServerStream {
+	if existing, ok := stream.(*WrappedServerStream); ok {
+		return existing
+	}
+	return &WrappedServerStream{ServerStream: stream, WrappedContext: stream.Context()}
+}