Histogram buckets do not adjust for unit

[ajatprabha]

Description

When I create a histogram without metric.WithUnit, it creates a prometheus histogram with buckets []float64{0, 5, 10, 25, 50, 75, 100, 250, 500, 750, 1000, 2500, 5000, 7500, 10000} from metric.DefaultAggregationSelector I believe.

But when I use metric.WithUnit("s"), the buckets do not re-adjust to account for this unit change. This should either be documented or handled by the SDK. For example: SDK handles naming in such case, i.e. _milliseconds or _seconds.

Environment

• OS: MacOS
• Architecture: arm64
• Go Version: 1.22.8
• opentelemetry-go version: v1.30.0

Expected behavior

When I use metric.WithUnit("s") which overrides the default Unit ms, it should adjust the buckets to []float64{0, 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1, 2.5, 5, 7.5, 10}, if not set explicitly.

[dmathieu]

Histogram buckets are defined by the specification.
https://github.com/open-telemetry/opentelemetry-specification/blob/0a78571045ca1dca48621c9648ec3c832c3c541c/specification/metrics/sdk.md#explicit-bucket-histogram-aggregation

Changing this behavior would require a specification change. Do you want to bring your use case there?

Note: I'm not sure this conversion would be the proper behavior. The default buckets come from defaults that make sense with ms. But if you change the unit, you likely want different values, not just a conversion by division.

[ajatprabha]

In my experience, when different values are needed, one updates the default buckets anyway.

Whether changing Unit should result in scaling of buckets or not, that's a good point of discussion but using the same bucket bounds meant for milliseconds also doesn't make sense I think.
I mean 5s,10s,25s,50s... are quite big buckets.

At the very least, it is better to document this in godoc comments. WDYT?

[dmathieu]

Yes, documenting this in godoc makes a lot of sense.