Details on converting exemplars for Prometheus

[anuraaga]

Currently we are missing some details for converting Prometheus exemplars. Some notable questions are

• Should we output timestamp? Out of order exemplars with Prometheus Exporter opentelemetry-java#4193

• What to do with extra attributes? https://github.com/open-telemetry/opentelemetry-java/pull/4178/files#r808247148

Originally posted by @anuraaga in #2266 (comment)

[irizzant]

@anuraaga is there any reason why this is closed?

[anuraaga]

Hi @irizzant - details were added here

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/datamodel.md#exemplars-1

[hanjm]

Hi @anuraaga , I have a question: why the no-histogram type exemplar dropped, the Prometheus exemplar is not limit to only histogram. thank you.

Exemplars
Exemplars on OpenTelemetry Histograms SHOULD be converted to OpenMetrics exemplars. Exemplars on other OpenTelemetry data points MUST be dropped.

[anuraaga]

Not sure - @dashpole what do you think?

[dashpole]

I think that statement was based on prometheus/prometheus#9738, which was mistaken to mean only histograms. Based on the openmetrics spec, exemplars can be added to counters in addition to histograms. I'll amend the spec to include support for exemplars from Otel monotonic sums (which are converted to openmetrics counters).