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

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

vendor/github.com/grpc-ecosystem/go-grpc-middleware/util/backoffutils/backoff.go

@@ -0,0 +1,23 @@
+// Copyright 2016 Michal Witkowski. All Rights Reserved.
+// See LICENSE for licensing terms.
+
+/*
+Backoff Helper Utilities
+
+Implements common backoff features.
+*/
+package backoffutils
+
+import (
+	"math/rand"
+	"time"
+)
+
+// JitterUp adds random jitter to the duration.
+//
+// This adds or substracts time from the duration within a given jitter fraction.
+// For example for 10s and jitter 0.1, it will returna  time within [9s, 11s])
+func JitterUp(duration time.Duration, jitter float64) time.Duration {
+	multiplier := jitter * (rand.Float64()*2 - 1)
+	return time.Duration(float64(duration) * (1 + multiplier))
+}