logdog: Add bundler library.

client/internal/logdog/butler/bundler/counter.go

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package bundler
+
+import (
+        "sync/atomic"
+)
+
+// counter is a goroutine-safe monotonically-increasing counter.
+type counter struct {
+        // current is the current counter value.
+        //
+        // It must be the first field in the struct to ensure it's 64-bit aligned
+        // for atomic operations.
+        current int64
+}
+
+func (c *counter) next() int64 {
+        // AddInt64 returns the value + 1, so subtract one to get its previous value
+        // (current counter value).
+        return atomic.AddInt64(&c.current, 1) - 1
+}