Regression test for cert-exporter metrics endpoint failing

[yulianedyalkova]

We had a regression in cert-exporter, slack thread:

cert_exporter_secret_not_after isn’t gone, but the whole cert-exporter /metrics endpoint is failing.

In v2.10.1 we started emitting one metric per certificate when a secret stores multiple certs concatenated in a single key.

Those samples ended up with identical labels, which makes Prometheus reject the scrape with HTTP 500 so all of the metric disappears, not just that secret’s :melting_face:

Confirmed it in tamarin-testing /metrics returns 500 with cert_exporter_secret_not_after … was collected before with the same name and label values, and 4 secrets trigger it.

Let's cover it with regression tests to ensure we don't encounter something similar again.