Append unit to prometheus metric names

[jack-berg]

The spec requires that unit be appended to the metric name as a suffix. We don't do that anywhere today. I believe that would represent a breaking behavior change for prometheus users as their metric names would change when unit is present.

Still, would be good to do that soon to minimize the impact.

Originally posted by @jack-berg in #4386 (comment)

[jack-berg]

Related spec issue.

[psx95]

I would like to take this up, if that's ok.
@jack-berg could you assign this to me ?

[psx95]

Just to confirm, since the link in the original description is broken, Prometheus and OpenMetrics Compatibility is the portion of the spec that needs to be followed to address this issue, correct ?

[dashpole]

That is correct. See https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#metric-metadata-1 for unit guidance.

[psx95]

Based on the semantic conventions in the spec document, I just wanted to confirm as to what should be the approach if user has a unit named {packets}, we cannot append this to the metric name, since it would violate the regex defined in the spec.

So in this case, we do drop the unit altogether, meaning no-op, is this correct ?

[dashpole]

That is considered unitless in prometheus, so we should drop units (at the portion within the brackets) with {}. I'll update the spec to more fully describe the mapping.