Ability to set CreatedTimestamp on constant histogram metrics

[swar8080]

#1313 added created timestamp to the internal histogram data model:

https://github.com/ArthurSens/client_golang/blob/e5639d1dcb98e3f1027e110d11ab9eda54e36f34/prometheus/histogram.go#L1202-L1218

I don't see a way for clients to set the value for created timestamp? Specifically we are trying to use prometheus.NewConstHistogram

I think this is why we're not getting any results when trying to calculate histogram_quantile since our _bucket counts don't start at zero without created timestamp injection. It'd also be nice if the _sum and _count metrics had zero injection.

---

Curious to know if hiding created timestamp from clients is intentional here, like if this part of the feature is WIP.

[ArthurSens]

Hmmmm, interesting. We have MustNewConstMetricWithCreatedTimestamp and even an example for it, but I just realize it doesn't cover Histograms.

I'm supportive of adding the same functionality for ConstHistograms and ConstSummaries.