Program.cs

using Prometheus;
using System.Diagnostics;

// This sample demonstrates how to attach exemplars to metrics exposed by a .NET console app.
// 
// NuGet packages required:
// * prometheus-net.AspNetCore

// Suppress some default metrics to make the output cleaner, so the exemplars are easier to see.
Metrics.SuppressDefaultMetrics(new SuppressDefaultMetricOptions
{
    SuppressEventCounters = true,
    SuppressMeters = true,
    SuppressProcessMetrics = true
});

// Start the metrics server on your preferred port number.
using var server = new KestrelMetricServer(port: 1234);
server.Start();

// Generate some sample data from fake business logic.
var recordsProcessed = Metrics.CreateCounter("sample_records_processed_total", "Total number of records processed.");
var recordSizeInPages = Metrics.CreateHistogram("sample_record_size_pages", "Size of a record, in pages.", new HistogramConfiguration
{
    Buckets = Histogram.PowersOfTenDividedBuckets(0, 2, 10)
});

// SAMPLED EXEMPLAR: For the next histogram we only want to record exemplars for values larger than 0.1 (i.e. when record processing goes slowly).
static Exemplar RecordExemplarForSlowRecordProcessingDuration(Collector metric, double value)
{
    if (value < 0.1)
        return Exemplar.None;

    return Exemplar.FromTraceContext();
}

var recordProcessingDuration = Metrics.CreateHistogram("sample_record_processing_duration_seconds", "How long it took to process a record, in seconds.", new HistogramConfiguration
{
    Buckets = Histogram.PowersOfTenDividedBuckets(-4, 1, 5),
    ExemplarBehavior = new()
    {
        DefaultExemplarProvider = RecordExemplarForSlowRecordProcessingDuration
    }
});

var totalSleepTime = Metrics.CreateCounter("sample_sleep_seconds_total", "Total amount of time spent sleeping.");

// CUSTOM EXEMPLAR: The key from an exemplar key-value pair should be created once and reused to minimize memory allocations.
var recordIdKey = Exemplar.Key("record_id");

_ = Task.Run(async delegate
{
    while (true)
    {
        // DEFAULT EXEMPLAR: We expose the trace_id and span_id for distributed tracing, based on Activity.Current.
        // Activity.Current is often automatically inherited from incoming HTTP requests if using OpenTelemetry tracing with ASP.NET Core.
        // Here, we manually create and start an activity for sample purposes, without relying on the platform managing the activity context.
        // See https://learn.microsoft.com/en-us/dotnet/core/diagnostics/distributed-tracing-concepts
        using (var activity = new Activity("Pausing before record processing").Start())
        {
            var sleepStopwatch = Stopwatch.StartNew();
            await Task.Delay(TimeSpan.FromSeconds(1));

            // The trace_id and span_id from the current Activity are exposed as the exemplar by default.
            totalSleepTime.Inc(sleepStopwatch.Elapsed.TotalSeconds);
        }

        using var processingDurationTimer = recordProcessingDuration.NewTimer();

        // Pretend to process a record approximately every second, just for changing sample data.
        var recordId = Guid.NewGuid();
        var recordPageCount = Random.Shared.Next(minValue: 5, maxValue: 100);

        // CUSTOM EXEMPLAR: We pass the record ID key-value pair when we increment the metric.
        // When the metric data is published to Prometheus, the most recent record ID will be attached to it.
        var exemplar = Exemplar.From(recordIdKey.WithValue(recordId.ToString()));

        // Note that one Exemplar object can only be used once. You must clone it to reuse it.
        recordsProcessed.Inc(exemplar.Clone());
        recordSizeInPages.Observe(recordPageCount, exemplar);
    }
});

// Metrics published in this sample:
// * the custom sample metrics defined above, with exemplars
// * internal debug metrics from prometheus-net, without exemplars
// Note that the OpenMetrics exposition format must be selected via HTTP header or query string parameter to see exemplars.
Console.WriteLine("Open http://localhost:1234/metrics?accept=application/openmetrics-text in a web browser.");
Console.WriteLine("Press enter to exit.");
Console.ReadLine();