 channel is mutual TLS and False otherwise. The `http` provided
        in the constructor will be overwritten.
        (2) If the environment variable is not set or `false`, the function does
        nothing and it always return False.

        Args:
            client_cert_callback (Optional[Callable[[], (bytes, bytes)]]):
                The optional callback returns the client certificate and private
                key bytes both in PEM format.
                If the callback is None, application default SSL credentials
                will be used.

        Returns:
            True if the channel is mutual TLS and False otherwise.

        Raises:
            google.auth.exceptions.MutualTLSChannelError: If mutual TLS channel
                creation failed for any reason.
        