What kind of checkers are available to be used by the check.
Values:
CHECKER_TYPE_UNSPECIFIED (0):
The default checker type. Currently converted to
STATIC_IP_CHECKERS on creation, the default conversion
behavior may change in the future.
STATIC_IP_CHECKERS (1):
STATIC_IP_CHECKERS are used for uptime checks that
perform egress across the public internet.
STATIC_IP_CHECKERS use the static IP addresses returned
by ListUptimeCheckIps.
VPC_CHECKERS (3):
VPC_CHECKERS are used for uptime checks that perform
egress using Service Directory and private network access.
When using VPC_CHECKERS, the monitored resource type
must be servicedirectory_service.
