Do not call out to Google on init

The GCP logging driver is calling out to GCP cloud service on package
init.
This is regardless if you are using GCP logging or not.

This change makes this happen on the first invocation of a new GCP
logging driver instance instead.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/logger/gcplogs/gcplogging.go

@@ -2,6 +2,7 @@ package gcplogs
 
 import (
 	"fmt"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -26,7 +27,7 @@ var (
 	// The number of logs the gcplogs driver has dropped.
 	droppedLogs uint64
 
-	onGCE = metadata.OnGCE()
+	onGCE bool
 
 	// instance metadata populated from the metadata server if available
 	projectID    string
@@ -36,16 +37,6 @@ var (
 )
 
 func init() {
-	if onGCE {
-		// These will fail on instances if the metadata service is
-		// down or the client is compiled with an API version that
-		// has been removed. Since these are not vital, let's ignore
-		// them and make their fields in the dockeLogEntry ,omitempty
-		projectID, _ = metadata.ProjectID()
-		zone, _ = metadata.Zone()
-		instanceName, _ = metadata.InstanceName()
-		instanceID, _ = metadata.InstanceID()
-	}
 
 	if err := logger.RegisterLogDriver(name, New); err != nil {
 		logrus.Fatal(err)
@@ -84,11 +75,30 @@ type containerInfo struct {
 	Metadata  map[string]string `json:"metadata,omitempty"`
 }
 
+var initGCPOnce sync.Once
+
+func initGCP() {
+	initGCPOnce.Do(func() {
+		onGCE = metadata.OnGCE()
+		if onGCE {
+			// These will fail on instances if the metadata service is
+			// down or the client is compiled with an API version that
+			// has been removed. Since these are not vital, let's ignore
+			// them and make their fields in the dockeLogEntry ,omitempty
+			projectID, _ = metadata.ProjectID()
+			zone, _ = metadata.Zone()
+			instanceName, _ = metadata.InstanceName()
+			instanceID, _ = metadata.InstanceID()
+		}
+	})
+}
+
 // New creates a new logger that logs to Google Cloud Logging using the application
 // default credentials.
 //
 // See https://developers.google.com/identity/protocols/application-default-credentials
 func New(ctx logger.Context) (logger.Logger, error) {
+	initGCP()
 
 	var project string
 	if projectID != "" {