Supporting timestamp in prometheus exporter

[kotharironak]

Discussed in #3696

^{Originally posted by kotharironak October 6, 2021}
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]

looking into this.

[jack-berg]

Closing with the merging of #3700.