Why Prometheus does not raise error on invisible metric?

Question

We have a custom metric that gets exported only upon some error condition in app

Alert rule use that custom metric that gets registered with rule manager of Prometheus

Why Prometheus does not raise error, when this metric name is queried? Despite the metric is not available in Prometheus yet...

Answer 1

It seems correct that the absence of a signal is not treated as an error.

However, it can cause problems with dashboards and alerting.

See this presentation by one of Prometheus' creators: Best Practices & Beastly Pitfalls