opentelemetry

name

opentelemetry

description

OpenTelemetry observability for .NET 10 applications. Covers traces, metrics, and logs using the OpenTelemetry SDK with OTLP export. Includes custom ActivitySource, IMeterFactory metrics, resource configuration, and Aspire Dashboard integration. Load this skill when setting up distributed tracing, custom metrics, OTLP export, or when the user mentions "OpenTelemetry", "OTLP", "traces", "spans", "Activity", "ActivitySource", "metrics", "IMeterFactory", "Meter", "Counter", "Histogram", "Gauge", "telemetry", "observability", "distributed tracing", "OTEL", or "Aspire Dashboard".

OpenTelemetry

Core Principles

1. Three pillars, one setup — Configure traces, metrics, and logs through a single AddOpenTelemetry() call. Use UseOtlpExporter() for cross-cutting export to any OTLP-compatible backend.
2. Use IMeterFactory for metrics — Never create Meter instances with new. The factory manages lifetime through DI and prevents leaks.
3. Null-safe activities — StartActivity() returns null when no listener is attached. Always use ?. when setting tags or events.
4. Environment variables over code — Use OTEL_EXPORTER_OTLP_ENDPOINT and OTEL_SERVICE_NAME so deployments control telemetry routing without code changes.
5. Low-cardinality metric tags — Keep metric tag combinations under ~1000 per instrument. Use span attributes or logs for high-cardinality data like user IDs or request IDs.

Patterns

Full Setup with All Three Signals

// Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Services.AddOpenTelemetry()
    .ConfigureResource(resource => resource
        .AddService(
            serviceName: builder.Environment.ApplicationName,
            serviceVersion: "1.0.0"))
    .WithTracing(tracing => tracing
        .AddAspNetCoreInstrumentation()
        .AddHttpClientInstrumentation()
        .AddEntityFrameworkCoreInstrumentation()
        .AddSource("MyApp.Orders"))
    .WithMetrics(metrics => metrics
        .AddAspNetCoreInstrumentation()
        .AddHttpClientInstrumentation()
        .AddRuntimeInstrumentation()
        .AddMeter("MyApp.Orders"))
    .WithLogging()             // no per-signal exporter here —
    .UseOtlpExporter();        // UseOtlpExporter covers all three signals

// UseOtlpExporter replaces per-signal AddOtlpExporter calls. Never combine
// the two — mixing them throws NotSupportedException (see Anti-patterns).

The OTLP endpoint defaults to http://localhost:4317 (gRPC). Override via:

OTEL_EXPORTER_OTLP_ENDPOINT=http://collector:4317
OTEL_SERVICE_NAME=MyApp.Api

Custom Metrics with IMeterFactory

Register a metrics class as a singleton. IMeterFactory handles Meter disposal through DI.

public sealed class OrderMetrics
{
    private readonly Counter<int> _ordersCreated;
    private readonly Histogram<double> _orderDuration;
    private readonly UpDownCounter<int> _activeOrders;
    private readonly Gauge<double> _queueDepth;

    public OrderMetrics(IMeterFactory meterFactory)
    {
        var meter = meterFactory.Create("MyApp.Orders");

        _ordersCreated = meter.CreateCounter<int>(
            "myapp.orders.created", "{orders}", "Number of orders created");

        _orderDuration = meter.CreateHistogram<double>(
            "myapp.orders.duration", "s", "Order processing duration",
            advice: new InstrumentAdvice<double>
            {
                HistogramBucketBoundaries = [0.01, 0.05, 0.1, 0.5, 1, 5, 10]
            });

        _activeOrders = meter.CreateUpDownCounter<int>(
            "myapp.orders.active", "{orders}", "Currently active orders");

        _queueDepth = meter.CreateGauge<double>(
            "myapp.orders.queue_depth", "{items}", );
    }

     => _ordersCreated.Add();
     => _orderDuration.Record(seconds);
     => _activeOrders.Add();
     => _activeOrders.Add();
     => _queueDepth.Record(depth);
}


builder.Services.AddSingleton<OrderMetrics>();

Multi-Dimensional Metric Tags

Three or fewer tags are allocation-free. For more, use TagList.

// Allocation-free (3 or fewer tags)
_ordersCreated.Add(1,
    new KeyValuePair<string, object?>("order.type", "standard"),
    new KeyValuePair<string, object?>("payment.method", "credit_card"));

// 4+ tags — use TagList to avoid allocations
var tags = new TagList
{
    { "order.type", "standard" },
    { "payment.method", "credit_card" },
    { "region", "us-east" },
    { "priority", "high" }
};
_ordersCreated.Add(1, tags);

Custom ActivitySource for Distributed Tracing

public sealed class OrderService(ILogger<OrderService> logger)
{
    private static readonly ActivitySource Source = new("MyApp.Orders");

    public async Task<Order> ProcessOrderAsync(CreateOrderRequest request, CancellationToken ct)
    {
        using var activity = Source.StartActivity("ProcessOrder", ActivityKind.Internal);
        activity?.SetTag("order.customer_id", request.CustomerId);

        try
        {
            await ValidateOrder(request, ct);
            activity?.AddEvent(new ActivityEvent("OrderValidated"));

            var order = await SaveOrder(request, ct);
            activity?.SetTag("order.id", order.Id.ToString());
            activity?.SetStatus(ActivityStatusCode.Ok);
            return order;
        }
        catch (Exception ex)
        {
            activity?.SetStatus(ActivityStatusCode.Error, ex.Message);
            activity?.RecordException(ex);
            throw;
        }
    }
}

Register the source: .AddSource("MyApp.Orders") in the tracing builder.

Aspire Dashboard for Local Development

Run the standalone Aspire Dashboard without Aspire orchestration:

docker run --rm -it -p 18888:18888 -p 4317:18889 \
    mcr.microsoft.com/dotnet/aspire-dashboard:latest

Then point your app at it:

OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4317

Dashboard UI is at http://localhost:18888.

Source-Generated Logging with OTel

For maximum performance, use [LoggerMessage] — eliminates boxing and allocations.

public partial class OrderService(ILogger<OrderService> logger)
{
    [LoggerMessage(Level = LogLevel.Information,
        Message = "Processing order {OrderId} for customer {CustomerId}")]
    partial void LogOrderProcessing(Guid orderId, Guid customerId);
}

OpenTelemetry logging automatically includes TraceId and SpanId when an Activity is current.

Anti-patterns

Don't Create Meters Per Request

// BAD — new Meter per request causes memory leaks
public void HandleRequest()
{
    var meter = new Meter("MyApp");
    meter.CreateCounter<int>("requests").Add(1);
}

// GOOD — singleton via IMeterFactory
public class MyMetrics(IMeterFactory meterFactory)
{
    private readonly Counter<int> _requests =
        meterFactory.Create("MyApp").CreateCounter<int>("myapp.requests");
    public void RequestHandled() => _requests.Add(1);
}

Don't Skip Null Checks on Activity

// BAD — NullReferenceException when no listener is attached
using var activity = source.StartActivity("Work");
activity.SetTag("key", "value");

// GOOD — null-safe
activity?.SetTag("key", "value");

Don't Use High-Cardinality Metric Tags

// BAD — unbounded cardinality causes memory explosion in collectors
_counter.Add(1, new("request.id", Guid.NewGuid().ToString()));
_counter.Add(1, new("user.id", userId));

// GOOD — low-cardinality dimensions only
_counter.Add(1, new("http.method", "GET"), new("http.status_code", 200));

Don't Mix UseOtlpExporter with AddOtlpExporter

// BAD — throws NotSupportedException at runtime
builder.Services.AddOpenTelemetry()
    .UseOtlpExporter()
    .WithTracing(t => t.AddOtlpExporter());

// GOOD — use one approach
builder.Services.AddOpenTelemetry().UseOtlpExporter();

Don't Forget to Register Custom Sources

// BAD — activities silently dropped (no listener registered)
var source = new ActivitySource("MyApp.Custom");
using var activity = source.StartActivity("Work"); // null!

// GOOD — register in the tracing builder
otel.WithTracing(t => t.AddSource("MyApp.Custom"));
otel.WithMetrics(m => m.AddMeter("MyApp.Custom"));

Decision Guide

Scenario	Recommendation
Full observability setup	AddOpenTelemetry() with all three signals + UseOtlpExporter()
Custom business metrics	IMeterFactory + singleton metrics class
Custom trace spans	ActivitySource + StartActivity()
Local development backend	Aspire Dashboard standalone container
Production backend	OTel Collector as intermediary to Grafana/Datadog/etc.
Sampling in production	OTEL_TRACES_SAMPLER=parentbased_traceidratio with 10% ratio
High-performance logging	[LoggerMessage] source generator
Metric tag cardinality	Max ~1000 combinations per instrument
Environment configuration	OTEL_* env vars (also work via appsettings.json)