Prometheus export from OTLP receiver not functional

[jmacd]

Describe the bug
Discussed under open-telemetry/opentelemetry-go#875, we have evidence that OTLP metrics data are being transformed incorrectly, such that the Prometheus exporter will:

• report Counter data as "gauge"
• drop Summary data (which the OTel-Go SDK is exporting for ValueRecorder, ValueObsever)

Steps to reproduce
The steps are described in open-telemetry/opentelemetry-go#875.

What did you expect to see?
Counter data should report as counter, ValueRecorder data should report as gauge.

What did you see instead?
Counters are labeled gauges, and no gauges appear.

What version did you use?
0.4

What config did you use?
This is also linked in open-telemetry/opentelemetry-go#875

Environment
Dockerized alpine

Additional context
I will self-assign this. I am working on it, just want it recorder here that Prometheus exporter is not functional with OTLP data due to a translation problem that I am investigating.

[huyan0]

I noticed the underlying prometheus-go-metric-exporter that the Collector's Prometheus exporter uses drop Summary when converting to Prometheus type. For some reason I thought it was a feature. Also, it doesn't care about the semantic difference and reports instantaneous adding scalar as a gauge.

Seems like the original prometheus-go-metric-exporter is moved to a different repo and support for Summary type is in progress .

[jmacd]

The big issue with prometheus-go-metric-exporter is that it uses the OpenCensus format, and we're migrating to OTLP. I am not certain what the best option is. A bigger question is which library will support Prometheus push: see #1150.

[jmacd]

I don't think we can sort this out until OTLP is settled, but it's fine to assign this to me.