Specifies how port is selected for health checking, can be one of
USE_FIXED_PORT: The port number in port is used for health checking.
USE_NAMED_PORT: The portName is used for health checking.
USE_SERVING_PORT: For NetworkEndpointGroup, the port specified for each
network endpoint is used for health checking. For other backends, the port
or named port specified in the Backend Service is used for health
The application data to send once the TCP connection has been established
(default value is empty). If both request and response are empty, the
connection establishment alone will indicate health. The request data can
only be ASCII.