Prometheus receiver doesn't support untyped metrics

[gracewehner]

Describe the bug
Prometheus receiver does not convert metrics without the TYPE hint to OTLP correctly and treats them as invalid, whereas Prometheus will treat these metrics as a gauge.

Steps to reproduce
Scrape an endpoint that has metrics without the # TYPE <metric name> <metric type> hint.

What did you expect to see?
Prometheus receiver matching Prometheus functionality and treating the metric as a gauge type.

[dashpole]

+1
I believe I encountered this as well when playing around with the bitnami spark helm chart as well.

I got a metric like metrics_master_aliveWorkers_Number{type="gauges"} 2 without a preceding type comment (and they put the type in a label?).

[dashpole]

This was discussed at the prometheus WG today in the context of additional compliance tests. One of the planned tests would check that MetricTypeUnknown metrics, including both ones you describe in the description, and metrics that are renamed (which is also currently a problem), are correctly propagated to prometheus remote_write endpoints from a prometheus receiver.

Correct me if i'm wrong, but I believe #4865 would change unknown typed metrics to gauges, and wouldn't pass such a test if we added it.

OpenCensus did support a MetricDescriptor_UNSPECIFIED type, which would allow this information to be propagated. pdata has a MetricDataTypeNone, but i'm not sure if that is the correct data type to use here.

cc @alolita @Aneurysm9

[dashpole]

These are the otlp data types: https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/metrics/v1/metrics.proto#L155-L181

[Aneurysm9]

I don't think MetricDataTypeNone will work, it's just the zero value for that type. There's no way to add or retrieve data with that type.

[github-actions]

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[Aneurysm9]

I believe this issue is no longer applicable. #6708 added tests that ensure we're handling untyped metrics as gauges. Please re-open if further issues are discovered.