telemetry: prevent stale values from cert monitors
telemetry: prevent stale values from cert monitors

Prometheus scrapes metrics from each process, so when leadership transfers to a different node
the previous leader would still be reporting the old cached value.

By setting NaN, I believe we should zero-out the value, so that prometheus should only consider the
value from the new leader.

hashicorp/consul,hashicorp/consul,hashicorp/consul,hashicorp/consul