Use OpenMetrics format for exposing exemplars

[bastianeicher]

Feature Request

The OpenMetrics specification extends the Prometheus format with - among other things - support for exemplars. All recent versions of Prometheus support this out of the box and it provides a great way to correlate metrics with traces.

I'd imagine an implementation like this:

• When a counter or histogram metric is updated, automatically capture Activity.Current (always keeping the latest value per metric).
• PrometheusExporterMiddleware and PrometheusExporterHttpServer check the Accept header of incoming scrape requests. If this contains application/openmetrics-text; version=1.0.0, append the capture activity/trace IDs to metrics as specified by OpenMetrics.

I believe this would provide great value because it would make integration between two of OpenTelemetry's pillars, tracing and metrics, just work (tm) out of the box.

If you feel that this would be a good fit and could give me some pointers as to the right places in the code to integrate such functionality, I'd be happy to whip up a proof-of-concept PR.

[cijothomas]

This feels like a specification issue. Every language implementations follow the OTel specification https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/prometheus.md

Also - Exemplars are not yet implemented in OpenTelemetry .NET

[bastianeicher]

The specification you linked to says:

A Prometheus Exporter MAY support OpenMetrics Text Format, including the Exemplars.

So basically my feature request would be to implement this optional part of the specification in OpenTelemetry .NET.

[cijothomas]

Yes. But Exemplars feature itself is not implemented.

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[bastianeicher]

I still think this would be a very valuable addition to opentelemetry-dotnet. This library supports both metrics and traces, so a technology for connecting them, namely exemplars, seems like a prefect fit.

[saul]

I've just raised #5929 for this