Create new health check to replace the one we lost when removing haystack-commons

[williewheeler]

The way the haystack-commons health check works is that when an uncaught exception occurs in Kafka, the health controller would write to a file indicating that the app is unhealthy, and then the k8s liveness check would see that and fail the health check.

We want something like this, but we need to coordinate it with #253 and #245. Ill-formatted metrics should now cause the health check to fail.