Support initial value in counters (Prometheus exporter)

[gaeljw]

From https://stackoverflow.com/questions/77349938/how-to-enforce-initial-value-zero-is-exported-to-prometheus-with-opentelemetry

Considering the following simple counter:

OpenTelemetry otel = ...

var meter = otel.getMeter("com.my.app");

LongCounter MyCounter = meter
  .counterBuilder("my_app_my_counter")
  .build();

As long as we don't increment the value of the counter (with .add(1, ...) for instance), the counter value is not visible to Prometheus scraping.

Using .add(0) at startup/initialization works as a trick, but shouldn't counters expose their initial value without having to do that? Or at least have an opt-in possibility to do it in the counter definition maybe?

Something like:

LongCounter MyCounter = meter
  .counterBuilder("my_app_my_counter")
  .withInitialValue(0) // <-- explicit initial value makes the value reported to Prometheus
  .build();

For completeness, here's the code we use to expose metrics to Prometheus thanks to OpenTelemetry Prometheus module:

import io.opentelemetry.exporter.prometheus.PrometheusHttpServer

PrometheusHttpServer prometheusServer = ...

SdkMeterProvider meterProvider = SdkMeterProvider
  .builder()
  .setResource(...)
  .registerMetricReader(prometheusServer)
  .build();

OpenTelemetrySdk openTelemetrySdk = OpenTelemetrySdk
  .builder()
  .setMeterProvider(meterProvider)
  .buildAndRegisterGlobal();

I don't know if this a core OpenTelemetry thing, or specific to Java, or even specific to the Prometheus Exporter.

[gaeljw]

Well, now that I give it another though, this might be tricky because of labels/attributes: we'd like initial values for some/all attributes but OpenTelemetry has no way to know them.

I guess that's the explanation.