Supporting timestamp in prometheus exporter

[kotharironak]

As per otel metrics proto, each metric's NumberDataPoint has a timestamp - [time_unix_nano](https://github.com/open-telemetry/opentelemetry-proto/blob/main/opentelemetry/proto/metrics/v1/metrics.proto#L376) - for recording the moment when the observation was aggregated.

But, in the Prometheus-exporter, this value is ignored while creating samples (link of createSample function). In my understanding, it is designed from sdk point of view and use in the application, so it will be a scrap moment when these metrics were collected.

I am looking into Kafka sink kind of a job where otlp metrics converted Prometheus format. I would like to re-use this exporter library. In that context, is there a way to support the time_unix_nano in this library while exporting? Can we make this library generic?

e.g Should we extend the signature of createSample to take timestamp?

Sample createSample(
    String name,
    List<String> labelNames,
    List<String> labelValues,
    double value,
    @Nullable ExemplarData exemplar,
long epochNanos)

[kotharironak]

@jsuereth what do you think?

[jsuereth]

As you've seen in that code, the exporter didn't intend to expose itself as anything but a normal prometheus exporter. As you saw, we assume that the prometheus collection timestamp is the same as the time_unix_nano we would have reported anyway.

How do you want to re-use this library? Are you calling the public methods on the prometheus exporter? Are you calling individual conversion methods? What is it you'd like exposed?

[kotharironak]

How do you want to re-use this library?

I am intent to use by creating an instance of PrometheusCollector by providing MetricProducer. I have a pipeline that receives metrics in otlp format, which are enriched with a few new labels in the process. Once, the enrichment is done, I need to export them to Prometheus. Here, I would like to reuse this library. So, I have written a MetricProducer. But, those metrics already have a timestamp (generated at the client end).

Are you calling the public methods on the prometheus exporter?

no

Are you calling individual conversion methods?

no

What is it you'd like exposed?

I am expecting that it should support timeStamp from MetricData points (PointData -> long getEpochNanos();).
Something like this - #3700