telemetry: improve cert expiry metrics
telemetry: improve cert expiry metrics

Emit the metric immediately so that after restarting an agent, the new expiry time will be
emitted. This is particularly important when this metric is being monitored, because we want
the alert to resovle itself immediately.

Also fixed a bug that was exposed in one of these metrics. The CARoot can be nil, so we have
to handle that case.

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