[exporter/prometheus] Should an undefined sum be exported as 0?

[pichlermarc]

As of open-telemetry/opentelemetry-collector#5530 (update to proto v0.18), sum is now optional and accessing returns 0. Therefore, if sum is not set, the prometheus exporter exports the sum as 0.

I'm not very familiar with Prometheus and I'm trying to determine if I should open a bug issue or if that is intentional. 🙂

[mx-psi]

cc @Aneurysm9 @dashpole

[dashpole]

An undefined sum should not be exported at all. IIRC, #9120 fixes this, but I need to rebase it as it got stale before it was reviewed.