When a NaN value is observed, the current implementation of histogram increases count and sets sum to NaN.

https://github.com/prometheus/client_golang/blob/main/prometheus/histogram_test.go#L541-L544

This violates two parts of the OpenMetrics spec:

• Sum MUST NOT be NaN.
• Count must be equal to the +Inf bucket.

This means NaN values must be ignored according to OpenMetrics.

I think ignoring NaN observations is a good idea, because then the count can be calculated from the bucket values. I'm doing this in client_java's new data model, and I like it because the derived count can never become inconsistent.

Anyway, it would be good if all Prometheus client libraries behaved the same, so what do you think of ignoring NaN observations?