Avoid ListBuckets() call instead rely on simple HTTP GET (#8475)

This is to avoid making calls to backend and requiring
gateways to allow permissions for ListBuckets() operation
just for Liveness checks, we can avoid this and make
our liveness checks to be more performant.

cmd/gateway/gcs/gateway-gcs.go

@@ -208,6 +208,9 @@ func (g *GCS) NewGatewayLayer(creds auth.Credentials) (minio.ObjectLayer, error)
 	gcs := &gcsGateway{
 		client:    client,
 		projectID: g.projectID,
+		httpClient: &http.Client{
+			Transport: minio.NewCustomHTTPTransport(),
+		},
 	}
 
 	// Start background process to cleanup old files in minio.sys.tmp
@@ -346,8 +349,9 @@ func isValidGCSProjectIDFormat(projectID string) bool {
 // gcsGateway - Implements gateway for MinIO and GCS compatible object storage servers.
 type gcsGateway struct {
 	minio.GatewayUnsupported
-	client    *storage.Client
-	projectID string
+	client     *storage.Client
+	httpClient *http.Client
+	projectID  string
 }
 
 // Returns projectID from the GOOGLE_APPLICATION_CREDENTIALS file.
@@ -412,8 +416,10 @@ func (l *gcsGateway) Shutdown(ctx context.Context) error {
 }
 
 // StorageInfo - Not relevant to GCS backend.
-func (l *gcsGateway) StorageInfo(ctx context.Context) minio.StorageInfo {
-	return minio.StorageInfo{}
+func (l *gcsGateway) StorageInfo(ctx context.Context) (si minio.StorageInfo) {
+	si.Backend.Type = minio.BackendGateway
+	si.Backend.GatewayOnline = minio.IsBackendOnline(ctx, l.httpClient, "https://storage.googleapis.com")
+	return si
 }
 
 // MakeBucketWithLocation - Create a new container on GCS backend.