Metrics endpoint returns internal server error if there are any certificate failures

[sboschman]

We run the latest version 3.17.0 and we notice that if there are any certificate failures the metrics endpoint throws an internal server error. Of course this means Prometheus is unable to scrape the pod.

Log:

level=INFO msg="Discovered and parsed certificates" certificates=22 failures=2

Exec into the pod:

/ $ wget localhost:9793
Connecting to localhost:9793 (127.0.0.1:9793)
wget: server returned error: HTTP/1.1 404 Not Found

/ $ wget localhost:9793/metrics
Connecting to localhost:9793 (127.0.0.1:9793)
wget: server returned error: HTTP/1.1 500 Internal Server Error

It seems to be a regression of #25 .