Exemplars support for Prometheus Counter

[jonatan-ivanov]

fixes gh-2811

[fscellos]

Hello @jonatan-ivanov , as i comment in #2812, it is necessary to take account sampled trace flags (https://www.w3.org/TR/trace-context/#sampled-flag) to be sure that the trace_id set in exemplars will be effectively recorded in backend storage.

[fscellos]

Hello, this is doable with OpenTelemetry API (Span.current().getSpanContext().getTraceFlags().isSampled()).
To use it, i think we must implement and use Samplers (ExemplarSampler) that take this information into account no new Examplar is this data is not set to true).
I will try.

[jonatan-ivanov]

@fscellos That would be the responsibility of the implementation of the SpanContextSupplier that we are not providing here. I don't see why would we need OTel for a Prometheus feature.

[fscellos]

I create an issue on prometheus client for this case : prometheus/client_java#765