logdog: Add bundler library.

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

Index: client/internal/logdog/butler/bundler/counter.go
diff --git a/client/internal/logdog/butler/bundler/counter.go b/client/internal/logdog/butler/bundler/counter.go
new file mode 100644
index 0000000000000000000000000000000000000000..48ad25767bb944455648fcf25755430aab4e0880
--- /dev/null
+++ b/client/internal/logdog/butler/bundler/counter.go
@@ -0,0 +1,24 @@
+// 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
+}