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
new file mode 100644
index 0000000000000000000000000000000000000000..75d09530534a780e3836a955541c787398274bad
--- /dev/null
+++ b/client/internal/logdog/butler/output/doc.go
@@ -0,0 +1,15 @@
+// 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 instances must be goroutine-safe. The Butler may elect to output

[estaab, 2015/11/19 23:57:24]

Wait, "must be" or "are"? I'm reading this is as: users can safely use Output
instances in goroutines but anyone changing this code must ensure it stays this
way. Is that right? It sort of looks like instructions to the user of the code
so perhaps disambiguate this statement by explicitly mentioning caller vs
author.

(and only here, not in the implementation files, it's much more obvious where
the audience is there)

[dnj, 2015/11/20 01:55:12]

Must. The Output interface contract promises goroutine-safety.

+// multiple messages at the same time.
+//
+// The package current provides the following implementations:
+//   - gcps: Write logs to Google Cloud Pub/Sub.

[estaab, 2015/11/19 23:57:24]

A better name might be just "pubsub" since it's instantly recognizable.

[dnj, 2015/11/20 01:55:12]

Hmm ... okay, agreed.

+//   - log: (Debug/testing) data is dumped to the installed Logger instance.
+package output