LogDog: Add collector service implementation.

client/internal/logdog/butler/output/pubsub/pubsubOutput.go

Index: client/internal/logdog/butler/output/pubsub/pubsubOutput.go
diff --git a/client/internal/logdog/butler/output/pubsub/pubsubOutput.go b/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
index cda2070f6b9ffa64ff19bb07a9286d52283fe77d..4910196b8ba744a7f14f15fb5e796f747c4f5857 100644
--- a/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
+++ b/client/internal/logdog/butler/output/pubsub/pubsubOutput.go
@@ -22,10 +22,20 @@ import (
 	"google.golang.org/cloud/pubsub"
 )
 
+// Publisher is an interface for something that publishes Pub/Sub messages.

[dnj (Google), 2016/01/21 04:36:24]

Make a specific interface instead of requiring one that implements all methods
when we only need Publish :)

+//
+// gcps.PubSub implements this interface.
+type Publisher interface {
+	// Publish mirrors the gcps.PubSub Publish method.
+	Publish(context.Context, gcps.Topic, ...*pubsub.Message) ([]string, error)
+}
+
+var _ Publisher = gcps.PubSub(nil)
+
 // Config is a configuration structure for GCPS output.
 type Config struct {
-	// Pubsub is the Pub/Sub instance to use.
-	PubSub gcps.PubSub
+	// Publisher is the Pub/Sub instance to use.
+	Publisher Publisher
 
 	// Topic is the name of the Cloud Pub/Sub topic to publish to.
 	Topic gcps.Topic
@@ -36,7 +46,7 @@ type Config struct {
 
 // Validate validates the Output configuration.
 func (c *Config) Validate() error {
-	if c.PubSub == nil {
+	if c.Publisher == nil {
 		return errors.New("gcps: no pub/sub instance configured")
 	}
 	if err := c.Topic.Validate(); err != nil {
@@ -165,7 +175,7 @@ func (o *gcpsOutput) publishMessages(messages []*pubsub.Message) error {
 	var messageIDs []string
 	count := 0
 	err := retry.Retry(o, retry.TransientOnly(retry.Default()), func() error {
-		ids, err := o.PubSub.Publish(o.Topic, messages...)
+		ids, err := o.Publisher.Publish(o, o.Topic, messages...)
 		if err != nil {
 			return err
 		}