Support exemplars in Prometheus exporter

[damemi]

Problem Statement

The Prometheus exporter in the new metrics SDK should be updated to support exemplars

Additional Context

Original PR for prometheus exporter: https://github.com/open-telemetry/opentelemetry-go/pull/3135/files#r966288412

[songjiayang]

really need this feature, thanks.

[MrAlias]

Blocked by adding exemplars to the SDK in #559

[MrAlias]

Discussed in the SIG meeting today: this might better fit in the post-GA metric SDK project. It needs to be verified that this can be added in a backwards compatible manner. If so, work to add it should be postponed until the specification has stabilized this section.

[kernelpanic77]

@MrAlias does this issue still require work ?

[MrAlias]

@kernelpanic77 it looks like it does:

opentelemetry-go/exporters/prometheus/exporter.go

Line 252 in d9d9507

// TODO(https://github.com/open-telemetry/opentelemetry-go/issues/3163): support exemplars

Should I assign it to you?

[MrAlias]

cc @dashpole

[kernelpanic77]

@kernelpanic77 it looks like it does:

opentelemetry-go/exporters/prometheus/exporter.go

Line 252 in d9d9507

// TODO(https://github.com/open-telemetry/opentelemetry-go/issues/3163): support exemplars

Should I assign it to you?

Sure I can have a look @MrAlias.
Although don't have much idea about the exporter. I'll get back to you with any queries I have.

[dashpole]

@kernelpanic77 let me know if you need any help with this, as it is fairly complex. For reference, the spec for this is: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/prometheus_and_openmetrics.md#exemplars-1

Exemplars on OpenTelemetry Histograms and Monotonic Sums SHOULD be converted to OpenMetrics exemplars. Exemplars on other OpenTelemetry data points MUST be dropped. ... For Prometheus pull endpoints, only a single exemplar is able to be added to each bucket, so the largest exemplar from each bucket MUST be used, if attaching exemplars. If no exemplars exist on a bucket, the highest exemplar from a lower bucket MUST be used, even though it is a duplicate of another bucket's exemplar. OpenMetrics Exemplars MUST use the trace_id and span_id keys for the trace and span IDs, respectively. Timestamps MUST be added as timestamps on the OpenMetrics exemplar, and filtered_attributes MUST be added as labels on the OpenMetrics exemplar unless they would exceed the OpenMetrics limit on characters.

[kernelpanic77]

Hi @dashpole,
Thanks for the reference, i'm having a look at the spec for this. I am still trying to get familiar with prometheus exporters and the otel-go lib. Can we discuss this in the otel-go SIG this week ?