OTLP Exporter does not set an observable counter as monotonic

[samin36]

Recently upgraded to Opentelemetry-cpp v1.12.0 and noticed some difference in how the OTLP Exporter exports Counters/UpDownCounters. In the previous version (v1.9.0) I was using, all observable Counters/UpDownCounters were exported with the is_monotonic flag set to true. As a result, the Prometheus exporter in the Opentelemetry Collector Contrib mapped the OTEL Counters/UpDownCounters to Prometheus Counters. Issue #2170 changed the behavior to only set the monotonic flag for kCounter. As a result, in v1.12.0, only the synchronous Counter is mapped to the Prometheus Counter type by the Collector; synchronous UpDownCounter and observable Counter/UpDownCounters are mapped to Prometheus Gauge type.

My question is, shouldn't observable Counters be mapped to Prometheus Counter type? For that to happen, we would need to set the is_monotonic flag for both kCounter and kObservableCounter. According to the official spec, if the aggregation temporality is cumulative and the sum is monotonic, it MUST be converted to a Prometheus Counter.

Also, for reference, seems like the Python SDK exports observable Counters with the is_monotonic flag set as the Prometheus endpoint shows the instrument type as a Counter instead of a Gauge.

[lalitb]

@samin36 your analysis is correct. Both Counter and ObservableCounter should have the is_monotonic flag as true. Do you want to raise a PR for the fix?

[samin36]

Thank you for confirming, and sure, will raise a PR soon.