LogDog: Add Butler Output package.

client/internal/logdog/butler/output/doc.go

Index: client/internal/logdog/butler/output/doc.go
diff --git a/client/internal/logdog/butler/output/doc.go b/client/internal/logdog/butler/output/doc.go
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index 0000000000000000000000000000000000000000..a4616525b7eb9a28557b3d6887a6c50edcc65979
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+++ b/client/internal/logdog/butler/output/doc.go
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+// 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 output contains interfaces and implementations for Butler Outputs,
+// which are responsible for delivering Butler protobufs to LogDog collection
+// endpoints.
+//
+// Output instance implementations must be goroutine-safe. The Butler may elect
+// to output multiple messages at the same time.
+//
+// The package current provides the following implementations:
+//   - pubsub: Write logs to Google Cloud Pub/Sub.
+//   - log: (Debug/testing) data is dumped to the installed Logger instance.
+package output