opentelemetry-dotnet/docs/metrics/extending-the-sdk/README.md

Extending the OpenTelemetry .NET SDK

• Building your own exporter
• Building your own reader
• Building your own exemplar reservoir
• Building your own resource detector
• References

Exporter

OpenTelemetry .NET SDK has provided the following built-in metric exporters:

• InMemory
• Console
• OpenTelemetryProtocol
• Prometheus HttpListener
• Prometheus AspNetCore

Custom exporters can be implemented to send telemetry data to places which are not covered by the built-in exporters:

• Exporters should derive from OpenTelemetry.BaseExporter<Metric> (which belongs to the OpenTelemetry package) and implement the Export method.
• Exporters can optionally implement the OnShutdown method.
• Exporters should not throw exceptions from Export and OnShutdown.
• Exporters are responsible for any retry logic needed by the scenario. The SDK does not implement any retry logic.
• Exporters should avoid generating telemetry and causing live-loop, this can be done via OpenTelemetry.SuppressInstrumentationScope.
• Exporters receives a batch of Metric, and each Metric can contain 1 or more MetricPoints. The exporter should perform all actions (e.g. serializing etc.) with the Metrics and MetricsPoints in the batch before returning control from Export, once the control is returned, the exporter can no longer make any assumptions about the state of the batch or anything inside it.
• Exporters should use ParentProvider.GetResource() to get the Resource associated with the provider.

class MyExporter : BaseExporter<Metric>
{
    public override ExportResult Export(in Batch<Metric> batch)
    {
        using var scope = SuppressInstrumentationScope.Begin();

        foreach (var metric in batch)
        {
            Console.WriteLine($"Export: {metric.metric}");

            foreach (ref readonly var metricPoint in metric.GetMetricPoints())
            {
                Console.WriteLine($"Export: {metricPoint.StartTime}");
            }
        }

        return ExportResult.Success;
    }
}

A demo exporter which simply writes metric name, metric point start time and tags to the console is shown here.

Apart from the exporter itself, you should also provide extension methods as shown here. This allows users to add the Exporter to the MeterProvider as shown in the example here.

Reader

Not supported.

ExemplarReservoir

[!NOTE] ExemplarReservoir is an experimental API only available in pre-release builds. For details see: OTEL1004. Please provide feedback to help inform decisions about what should be exposed stable and when.

Custom ExemplarReservoirs can be implemented to control how Exemplars are recorded for a metric:

• ExemplarReservoirs should derive from FixedSizeExemplarReservoir (which belongs to the OpenTelemetry package) and implement the Offer methods.
• The FixedSizeExemplarReservoir constructor accepts a capacity parameter to control the number of Exemplars which may be recorded by the ExemplarReservoir.
• The virtual OnCollected method is called after the ExemplarReservoir collection operation has completed and may be used to implement cleanup or reset logic.
• The bool ResetOnCollect property on ExemplarReservoir is set to true when delta aggregation temporality is used for the metric using the ExemplarReservoir.
• The Offer and Collect ExemplarReservoir methods are called concurrently by the OpenTelemetry SDK. As such any state required by custom ExemplarReservoir implementations needs to be managed using appropriate thread-safety/concurrency mechanisms (lock, Interlocked, etc.).
• Custom ExemplarReservoir implementations MUST NOT throw exceptions. Exceptions thrown in custom implementations MAY lead to unreleased locks and undefined behaviors.

The following example demonstrates a custom ExemplarReservoir implementation which records Exemplars for measurements which have the highest value. When delta aggregation temporality is used the recorded Exemplar will be the highest value for a given collection cycle. When cumulative aggregation temporality is used the recorded Exemplar will be the highest value for the lifetime of the process.

class HighestValueExemplarReservoir : FixedSizeExemplarReservoir
{
    private readonly object lockObject = new();
    private long? previousValueLong;
    private double? previousValueDouble;

    public HighestValueExemplarReservoir()
        : base(capacity: 1)
    {
    }

    public override void Offer(in ExemplarMeasurement<long> measurement)
    {
        if (!this.previousValueLong.HasValue || measurement.Value > this.previousValueLong.Value)
        {
            lock (this.lockObject)
            {
                if (!this.previousValueLong.HasValue || measurement.Value > this.previousValueLong.Value)
                {
                    this.UpdateExemplar(0, in measurement);
                    this.previousValueLong = measurement.Value;
                }
            }
        }
    }

    public override void Offer(in ExemplarMeasurement<double> measurement)
    {
        if (!this.previousValueDouble.HasValue || measurement.Value > this.previousValueDouble.Value)
        {
            lock (this.lockObject)
            {
                if (!this.previousValueDouble.HasValue || measurement.Value > this.previousValueDouble.Value)
                {
                    this.UpdateExemplar(0, in measurement);
                    this.previousValueDouble = measurement.Value;
                }
            }
        }
    }

    protected override void OnCollected()
    {
        if (this.ResetOnCollect)
        {
            lock (this.lockObject)
            {
                this.previousValueLong = null;
                this.previousValueDouble = null;
            }
        }
    }
}

Custom ExemplarReservoirs can be configured using the View API. For details see: Change the ExemplarReservoir.

References