// Should only be modified before any calls are made.
	MaxErrResponseSize int64

	// Avoid metrics update if set to true
	NoMetrics bool

	// TraceOutput will print debug information on non-200 calls if set.
	TraceOutput io.Writer // Debug trace output

	httpClient   *http.Client
	url          *url.URL
	newAuthToken func(audience string) string

	sync.RWMutex // mutex for lastErr
	lastErr      error