Specifies how a port is selected for health checking. Can be one of
the following values: USE_FIXED_PORT: Specifies a port number
explicitly using the port field in the health check. Supported by
backend services for pass-through load balancers and backend
services for proxy load balancers. Not supported by target pools.
The health check supports all backends supported by the backend
service provided the backend can be health checked. For example,
GCE_VM_IP network endpoint groups, GCE_VM_IP_PORT network endpoint
groups, and instance group backends. USE_NAMED_PORT: Not supported.
USE_SERVING_PORT: Provides an indirect method of specifying the
health check port by referring to the backend service. Only
supported by backend services for proxy load balancers. Not
supported by target pools. Not supported by backend services for
pass-through load balancers. Supports all backends that can be
health checked; for example, GCE_VM_IP_PORT network endpoint groups
and instance group backends. For GCE_VM_IP_PORT network endpoint
group backends, the health check uses the port number specified for
each endpoint in the network endpoint group. For instance group
backends, the health check uses the port number determined by
looking up the backend service's named port in the instance group's
list of named ports.