Request the project ID from the metadata server only once

If there is an error such as "no route to host", which comes up during
development, every request will slow down. Now the server is only
contacted once. This will speed up requests in production as well by
removing the call to the metadata server from all but the first request.

gaelog.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"os"
 	"strings"
+	"sync"
 	"time"
 
 	"cloud.google.com/go/compute/metadata"
@@ -31,6 +32,22 @@ const (
 	traceContextHeaderName = "X-Cloud-Trace-Context"
 )
 
+var (
+	metadataOnce sync.Once
+
+	metadataProjectID    string
+	metadataProjectIDErr error
+)
+
+// projectIDFromMetadataService fetches the project ID from the metadata server,
+// memoizing the result for use on all but the first call.
+func projectIDFromMetadataService() (string, error) {
+	metadataOnce.Do(func() {
+		metadataProjectID, metadataProjectIDErr = metadata.ProjectID()
+	})
+	return metadataProjectID, metadataProjectIDErr
+}
+
 func traceID(projectID, trace string) string {
 	return fmt.Sprintf("projects/%s/traces/%s", projectID, trace)
 }
@@ -64,7 +81,7 @@ func newServiceInfo() (serviceInfo, error) {
 	}
 
 	// Try the metadata service for the project ID.
-	crProjectID, err := metadata.ProjectID()
+	crProjectID, err := projectIDFromMetadataService()
 	if err != nil {
 		return serviceInfo{}, err
 	}