How to use Gauge metric instrument in c++ opentelemetry with otlp http/grpc exporter?

Question

In my c++ project I use opentelemetry (v1.14.2) to collect traces and logs. Now I want to add metrics collection using Gauge instrument (double observable gauge) with otlp http/grpc exporter. I've read the official documentation and many other resources and didn't find any gauge usage example.

I tried following code:

void InitMetrics(const std::string& name)
{
  opentelemetry::exporter::otlp::OtlpHttpMetricExporterOptions opts;
  opts.url = "http://ip:port/v1/metrics";
  auto exporter = std::make_unique<otlpexportmetrics::OtlpHttpMetricExporter>(opts);

  metric_sdk::PeriodicExportingMetricReaderOptions options;
  options.export_interval_millis = std::chrono::milliseconds(1000);
  options.export_timeout_millis = std::chrono::milliseconds(500);
  std::shared_ptr<metric_sdk::MetricReader> reader{
      new metric_sdk::PeriodicExportingMetricReader(std::move(exporter),
                                                    options)};
  auto provider = std::shared_ptr<metrics_api::MeterProvider>(
      new metric_sdk::MeterProvider());
  auto p = std::static_pointer_cast<metric_sdk::MeterProvider>(provider);
  p->AddMetricReader(std::move(reader));
}

void GaugeExample(const std::string& name)
{
  std::string gauge_name = name + "_gauge";
  auto meter = metrics_api::Provider::GetMeterProvider()->GetMeter(name+"_meter", "1.0.0", "");
  auto gauge = meter->CreateDoubleObservableGauge(gauge_name, "pressure", "MPa");
  gauge->AddCallback([](opentelemetry::metrics::ObserverResult res, void* state){
                       nostd::shared_ptr<opentelemetry::metrics::ObserverResultT<double>> obs;
                       obs->Observe(5.6, {{"service.name", "example"}});
                       res.emplace<nostd::shared_ptr<opentelemetry::metrics::ObserverResultT<double>>>(obs);
                       }, nullptr);
}

int main()
{
  InitMetrics("example");
  GaugeExample("example");
  std::this_thread::sleep_for(std::chrono::seconds(20));
  std::shared_ptr<metrics_api::MeterProvider> none;
  metrics_api::Provider::SetMeterProvider(none);
  return 0;
}

I have SigNoz set up with otlp collector and want to see metrics there (logs and traces exports there well and endpoint is 100% correct) but my metrics don't even go to the otlp collector and I have following warning executing my program:

[Warning] File: /home/user/vcpkg/buildtrees/opentelemetry-cpp/src/v1.14.2-8c1c1d9b6b.clean/sdk/src/metrics/metric_reader.cc:37 MetricReader::Collect invoked while Shutdown in progress!

Could someone give me any working example of Gauge usage?

Answer 1

Gauge metric is observable metric, and it's a bit simular to ObservableCounter, so this may help (observable_counter_example): https://github.com/open-telemetry/opentelemetry-cpp/blob/main/examples/common/metrics_foo_library/foo_library.cc