How to export OpenCensus metrics with OpenCensusShim and PrometheusHttpServer

[jianwu]

Based on this doc: https://github.com/open-telemetry/opentelemetry-java/tree/main/opencensus-shim. We can export OC metrics by register the MetricsReader as shown in the example:

LoggingMetricExporter metricExporter = LoggingMetricExporter.create();
SdkMeterProvider.builder()
    .registerMetricReader(OpenCensusMetrics.attachTo(PeriodicMetricReader.create(metricExporter)))
    .build();

However as we are using PrometheusHttpServer exporter. A PrometheusHttpsService instance is automatically created with service provider: https://github.com/open-telemetry/opentelemetry-java/blob/main/exporters/prometheus/src/main/resources/META-INF/services/io.opentelemetry.sdk.autoconfigure.spi.AutoConfigurationCustomizerProvider
once I introduced the dependency to this module. In that case, I can't get a reference of this PrometheusHttpsService. And I also can't create a new one, as it will have port conflict. How can I get the existing PrometheusHttpsService instance registered?

[jianwu]

I figured out to use agent extension to customize the metrics exporter with following code in the agent extension:

public class CustomizeProvider implements AutoConfigurationCustomizerProvider {
    @Override
  public void customize(AutoConfigurationCustomizer autoConfigurationCustomizer) {
    autoConfigurationCustomizer.addMeterProviderCustomizer(this::buildMeterProvider);
  }

  SdkMeterProviderBuilder buildMeterProvider(SdkMeterProviderBuilder meterProviderBuilder, ConfigProperties config) {
    log("Register OpenCensusMetricProducer shim:");
    meterProviderBuilder.registerMetricProducer(OpenCensusMetricProducer.create());
    return meterProviderBuilder;
  }
}

However, I noticed seems in agent, the class is loaded by a different classloader. The OpenCensusMetricProducer.create() is calling Metrics.getExportComponent().getMetricProducerManager(). But this method will return a different instance than the one used by application. So it can't see the registered metrics generated by application.

How can I bypass this issue.
Related dicussion: open-telemetry/opentelemetry-java-instrumentation#7042

[jianwu]

Is it possible to give an example of using metrics shim with agent and auth-configuration in Java?