Specifies the key used for connection tracking. There are two
options: - PER_CONNECTION: This is the default mode. The Connection
Tracking is performed as per the Connection Key (default Hash
Method) for the specific protocol. - PER_SESSION: The Connection
Tracking is performed as per the configured Session Affinity. It
matches the configured Session Affinity. For more details, see
Tracking Mode for Network Load
Balancing <https://cloud.google.com/load-balancing/docs/network/networklb-backend-service#tracking-mode>
and Tracking Mode for Internal TCP/UDP Load
Balancing <https://cloud.google.com/load-balancing/docs/internal#tracking-mode>.