The current /exit method is not sufficient to test graceful shutdown
behaviors within Kube that allow services to remain available during
rolling restarts. Add support for `wait=DURATION` and
`timeout=DURATION` to the exit handler and wire that to the Go http
server's graceful termination.
With these methods netexec can be used in a pod to simulate graceful
shutdown by adding a preStop handler that hits the exit endpoint with
a timeout and wait period.