be used to create a stub that can make authorized requests.
    Users can configure client certificate or rely on device certificates to
    establish a mutual TLS channel, if the `GOOGLE_API_USE_CLIENT_CERTIFICATE`
    variable is explicitly set to `true`.

    Example::

        import google.auth
        import google.auth.transport.grpc
        import google.auth.transport.requests
        from google.cloud.speech.v1 import cloud_speech_pb2

        # Get credentials.
        credentials, _ = google.auth.default()

        # Get an HTTP request function to refresh credentials.
        request = google.auth.transport.requests.Request()

        # Create a channel.
        channel = google.auth.transport.grpc.secure_authorized_channel(
            credentials, regular_endpoint, request,
            ssl_credentials=grpc.ssl_channel_credentials())

        # Use the channel to create a stub.
        cloud_speech.create_Speech_stub(channel)

    Usage:

    There are actually a couple of options to create a channel, depending on if
    you want to create a regular or mutual TLS channel.

    First let's list the endpoints (regular vs mutual TLS) to choose from::

        regular_endpoint = 'speech.googleapis.com:443'
        mtls_endpoint = 'speech.mtls.googleapis.com:443'

    Option 1: create a regular (non-mutual) TLS channel by explicitly setting
    the ssl_credentials::

        regular_ssl_credentials = grpc.ssl_channel_credentials()

        channel = google.auth.transport.grpc.secure_authorized_channel(
            credentials, regular_endpoint, request,
            ssl_credentials=regular_ssl_credentials)

    Option 2: create a mutual TLS channel by calling a callback which returns
    the client side certificate and the key (Note that
    `GOOGLE_API_USE_CLIENT_CERTIFICATE` environment variable must be explicitly
    set to `true`)::

        def my_client_cert_callback():
            code_to_load_client_cert_and_key()
            if loaded:
                return (pem_cert_bytes, pem_key_bytes)
            raise MyClientCertFailureException()

        try:
            channel = google.auth.transport.grpc.secure_authorized_channel(
                credentials, mtls_endpoint, request,
                client_cert_callback=my_client_cert_callback)
        except MyClientCertFailureException:
            # handle the exception

    Option 3: use application default SSL credentials. It searches and uses
    the command in a context aware metadata file, which is available on devices
    with endpoint verification support (Note that
    `GOOGLE_API_USE_CLIENT_CERTIFICATE` environment variable must be explicitly
    set to `true`).
    See https://cloud.google.com/endpoint-verification/docs/overview::

        try:
            default_ssl_credentials = SslCredentials()
        except:
            # Exception can be raised if the context aware metadata is malformed.
            # See :class:`SslCredentials` for the possible exceptions.

        # Choose the endpoint based on the SSL credentials type.
        if default_ssl_credentials.is_mtls:
            endpoint_to_use = mtls_endpoint
        else:
            endpoint_to_use = regular_endpoint
        channel = google.auth.transport.grpc.secure_authorized_channel(
            credentials, endpoint_to_use, request,
            ssl_credentials=default_ssl_credentials)

    Option 4: not setting ssl_credentials and client_cert_callback. For devices
    without endpoint verification support or `GOOGLE_API_USE_CLIENT_CERTIFICATE`
    environment variable is not `true`, a regular TLS channel is created;
    otherwise, a mutual TLS channel is created, however, the call should be
    wrapped in a try/except block in case of malformed context aware metadata.

    The following code uses regular_endpoint, it works the same no matter the
    created channle is regular or mutual TLS. Regular endpoint ignores client
    certificate and key::

        channel = google.auth.transport.grpc.secure_authorized_channel(
            credentials, regular_endpoint, request)

    The following code uses mtls_endpoint, if the created channle is regular,
    and API mtls_endpoint is confgured to require client SSL credentials, API
    calls using this channel will be rejected::

        channel = google.auth.transport.grpc.secure_authorized_channel(
            credentials, mtls_endpoint, request)

    Args:
        credentials (google.auth.credentials.Credentials): The credentials to
            add to requests.
        request (google.auth.transport.Request): A HTTP transport request
            object used to refresh credentials as needed. Even though gRPC
            is a separate transport, there's no way to refresh the credentials
            without using a standard http transport.
        target (str): The host and port of the service.
        ssl_credentials (grpc.ChannelCredentials): Optional SSL channel
            credentials. This can be used to specify different certificates.
            This argument is mutually exclusive with client_cert_callback;
            providing both will raise an exception.
            If ssl_credentials and client_cert_callback are None, application
            default SSL credentials are used if `GOOGLE_API_USE_CLIENT_CERTIFICATE`
            environment variable is explicitly set to `true`, otherwise one way TLS
            SSL credentials are used.
        client_cert_callback (Callable[[], (bytes, bytes)]): Optional
            callback function to obtain client certicate and key for mutual TLS
            connection. This argument is mutually exclusive with
            ssl_credentials; providing both will raise an exception.
            This argument does nothing unless `GOOGLE_API_USE_CLIENT_CERTIFICATE`
            environment variable is explicitly set to `true`.
        kwargs: Additional arguments to pass to :func:`grpc.secure_channel`.

    Returns:
        grpc.Channel: The created gRPC channel.

    Raises:
        google.auth.exceptions.MutualTLSChannelError: If mutual TLS channel
            creation failed for any reason.
    zUReceived both ssl_credentials and client_cert_callback; these are mutually exclusive.