add http health checks.

What about other non-error status codes?

Additionally, it would be convenient to be able to use the same handler as the load balancer health (readiness) checks. However, those checks return error codes (e.g., 503) during temporary unavailability. 

Also, errors due to misconfiguration are unlikely to be resolved by killing and restarting a container.

Since liveness probes are mainly a safeguard against deadlock, livelock, excessive queue lengths, etc., typically any response could be interpreted as "live".