OpenTelemetry .NET Instrumentation Skill

Description

Provides guidance for implementing OpenTelemetry instrumentation in .NET codebases, covering tracing (Activities/Spans), metrics, naming conventions, error handling, performance, and API design best practices.

When to Use

• Adding OpenTelemetry instrumentation to .NET code
• Creating or modifying ActivitySources and metrics
• Reviewing telemetry implementations for compliance
• Optimizing instrumentation performance
• Designing telemetry APIs that become part of the public surface

Prerequisites

• .NET application with OpenTelemetry SDK
• Understanding of System.Diagnostics.Metrics and ActivitySource APIs
• Access to observability backend (e.g., Jaeger, Prometheus, Grafana)

Core Principles

Resiliency First

CRITICAL: Exceptions in diagnostic/tracing/metrics logic MUST NEVER impact application processing.

• Always protect against null Activity references except in Activity extension methods (use activity?.ExtensionMethod())
• Assume Activity instances can be null (only created when listeners subscribe)
• Guard all instrumentation code with appropriate null checks

API Surface Awareness

• Any telemetry emitted becomes part of the public API surface
• Changes are subject to breaking changes guidelines
• Telemetry should be emitted by default (users opt-in to collection via OpenTelemetry extensions)
• Exception: High-cardinality metric dimensions may require explicit opt-in

Standards Compliance

• Follow Microsoft best practices for distributed tracing instrumentation
• Follow OpenTelemetry semantic conventions
• All attributes must be non-null, non-empty strings

Traces / Spans (Activities)

ActivitySource Setup

// ✅ CORRECT: Use ActivitySource, not DiagnosticSource
public class MyFeature
{
    // Primary ActivitySource - name typically matches the component or NuGet package name
    private static readonly ActivitySource ActivitySource = new("MyApp.MyComponent", "1.0.0");

    // Specialized ActivitySource for opt-in scenarios
    private static readonly ActivitySource DetailedActivitySource = new("MyApp.MyComponent.Detailed", "1.0.0");
}

Rules:

• Every component defines a primary ActivitySource for mainstream activities
• Name typically matches the component or NuGet package (e.g., "MyCompany.MyLibrary")
• Version the ActivitySource using SemVer
• Create separate ActivitySources for specialized/opt-in scenarios

Creating Activities

// ✅ CORRECT: Check HasListeners before creating
if (ActivitySource.HasListeners())
{
    using var activity = ActivitySource.StartActivity("ProcessItem", ActivityKind.Internal);

    if (activity != null)
    {
        activity.DisplayName = "Processing order #12345";

        // Only compute expensive tags if requested
        if (activity.IsAllDataRequested)
        {
            activity.SetTag("app.item_id", itemId);
            activity.SetTag("app.item_type", itemType);
        }
    }
}

// ❌ WRONG: Don't start activities in async helper methods (breaks AsyncLocal)
async Task HelperAsync()
{
    using var activity = ActivitySource.StartActivity("Helper"); // ❌ BAD
    await DoWorkAsync();
}

Rules:

• Check ActivitySource.HasListeners() before creating (zero-allocation fast path)
• Always check if activity is null after creation
• Never start activities in asynchronous helper methods (Activity.Current uses AsyncLocal)
• Use activity.IsAllDataRequested before expensive computations
• Always use W3C ID format (enforce format change if parent uses hierarchical)

Activity Naming

// ✅ CORRECT: Unique operation name, friendly display name
using var activity = ActivitySource.StartActivity(
    name: "ProcessItem",              // Unique, identifies class of spans
    kind: ActivityKind.Internal
);
activity.DisplayName = "Processing order #12345"; // User-friendly, can be specific

// ❌ WRONG: Don't include runtime data in operation name
using var activity = ActivitySource.StartActivity($"Process_{itemId}"); // ❌ BAD

Rules:

• Each span type has unique OperationName (identifies statistically interesting class of spans)
• Operation name should NOT contain runtime data (only compile/config-time info)
• Use human-readable DisplayName for specifics
• Follow OpenTelemetry span naming conventions

Span Attributes (Tags)

// ✅ CORRECT: Namespace, lowercase, underscore-delimited
activity?.SetTag("myapp.order_id", orderId);
activity?.SetTag("myapp.order_type", orderType);
activity?.SetTag("myapp.db.table_name", tableName);

// Standard semantic conventions where applicable
activity?.SetTag("db.system", "postgresql");
activity?.SetTag("http.method", "GET");

// ❌ WRONG: Various naming violations
activity?.SetTag("MyApp.OrderId", orderId);         // ❌ Wrong case
activity?.SetTag("myapp.order-id", orderId);        // ❌ Wrong delimiter
activity?.SetTag("myapp.orders", count);            // ❌ Plural
activity?.SetTag("unrelated.ip_address", ip);       // ❌ Not characteristic

Naming Conventions:

• Use a namespace prefix matching your component: myapp.*, myapp.db.*
• All lowercase letters
• Underscore (_) delimiters for multi-word attributes
• Singular form
• Only set tags directly relevant to this activity
• Prefer standard OpenTelemetry semantic conventions over custom attributes where they exist
• Only use standard semantic conventions if certain no downstream library will set them

Activity Status and Errors

// ✅ CORRECT: Set status and record exceptions
try
{
    await ProcessItemAsync();
    activity?.SetStatus(ActivityStatusCode.Ok);
}
catch (Exception ex)
{
    if (activity != null)
    {
        activity.SetStatus(ActivityStatusCode.Error);
        activity.SetTag("otel.status_code", "error");
        activity.SetTag("otel.status_description", ex.Message);

        // Record exception event per OTel spec
        activity.AddEvent(new ActivityEvent(
            "exception",
            tags: new ActivityTagsCollection
            {
                ["exception.type"] = ex.GetType().FullName,
                ["exception.message"] = ex.Message,
                ["exception.stacktrace"] = ex.ToString()
            }
        ));
    }
    throw;
}

Rules:

• Set ActivityStatusCode.Ok on success
• Set ActivityStatusCode.Error on exception
• Always add otel.status_code and otel.status_description tags
• Record exception events following OTel exception conventions

Activity Events

// ✅ CORRECT: Use events for additional context (sparingly)
activity?.AddEvent(new ActivityEvent("ItemRetried", tags: new ActivityTagsCollection
{
    ["retry_attempt"] = retryCount,
    ["next_retry_delay"] = delayMs
}));

// ❌ WRONG: Don't use events for verbose logging
activity?.AddEvent(new ActivityEvent($"Step {i} completed")); // ❌ Use logging instead

Rules:

• Events stored in-memory until transmission (use sparingly)
• Only for additional context; consider nested spans for multiple events
• Use logging for verbose information

Accessing Activities

// ❌ WRONG: Don't rely on Activity.Current when you need a specific span
public async Task HandleAsync(Context context)
{
    var activity = Activity.Current; // ❌ Might be a user-created span, not yours
    activity?.SetTag("custom", "value");
}

// ✅ CORRECT: Pass Activity explicitly or store it in a dedicated context object
public async Task HandleAsync(Context context)
{
    if (context.TryGetActivity(out var activity))
    {
        activity?.SetTag("custom", "value");
    }
}

Metrics

Meter and Metrics Class Setup

// ✅ CORRECT: Group metrics by feature/component
public sealed class OrderProcessingMetrics : IDisposable
{
    private readonly Meter meter;
    private readonly Histogram<double> processingDuration;
    private readonly Counter<long> itemsProcessed;

    public OrderProcessingMetrics()
    {
        meter = new Meter("MyApp.OrderProcessing", "1.0.0");

        // Singular names, appropriate units, nested hierarchy
        processingDuration = meter.CreateHistogram<double>(
            "myapp.order.processing.duration",
            unit: "s",
            description: "Duration of order processing"
        );

        itemsProcessed = meter.CreateCounter<long>(
            "myapp.order.processing.count",
            unit: "{order}",
            description: "Number of orders processed"
        );
    }

    public void Dispose() => meter.Dispose();
}

Naming Conventions (follow OTel semantic conventions):

• Singular names (use _count suffix instead of pluralization)
• Nested hierarchy: myapp.order.processing.duration
• Define units (s, ms, {item}, {connection})
• Avoid technical suffixes (_counter, _histogram)
• Start with pre-1.0.0 version until adoption proven

Metric Recording Method Naming

// ✅ CORRECT: Action/outcome-based naming, separate methods per outcome
public sealed class OrderProcessingMetrics
{
    // Event happened: describe what occurred
    public void OrderProcessingSucceeded(string orderType, TimeSpan duration)
    {
        processingDuration.Record(duration.TotalSeconds,
            new KeyValuePair<string, object?>("myapp.order_type", orderType),
            new KeyValuePair<string, object?>("outcome", "success")
        );
    }

    public void OrderProcessingFailed(string orderType, Exception exception, TimeSpan duration)
    {
        processingDuration.Record(duration.TotalSeconds,
            new KeyValuePair<string, object?>("myapp.order_type", orderType),
            new KeyValuePair<string, object?>("outcome", "failure"),
            new KeyValuePair<string, object?>("exception.type", exception.GetType().Name)
        );
    }

    public void ConnectionOpened() => connectionsOpen.Add(1);
    public void ConnectionClosed() => connectionsOpen.Add(-1);
}

// ❌ WRONG: Various naming anti-patterns
public void RecordOrderProcessingDuration(...) { } // ❌ Don't name after metric
public void RecordError(bool succeeded, Exception? ex) { } // ❌ Confusing signature

Rules (inspired by ASP.NET Core patterns):

• Name after action/outcome: OrderProcessingSucceeded, RetryAttempted, ConnectionFailed
• NOT after metric name: avoid RecordXxx, IncrementXxx
• Separate methods for different outcomes (avoid boolean flags + optional exceptions)
• Event-based naming for state changes: ConnectionOpened(), ItemQueued()

Metric Dimensions

// ✅ CORRECT: Low-cardinality, predefined dimensions
public void OrderProcessingSucceeded(string orderType, TimeSpan duration)
{
    processingDuration.Record(duration.TotalSeconds,
        new KeyValuePair<string, object?>("myapp.order_type", orderType),
        new KeyValuePair<string, object?>("myapp.region", region),
        new KeyValuePair<string, object?>("outcome", "success")
    );
}

// ❌ WRONG: High-cardinality dimensions (unbounded values cause cardinality explosion)
public void OrderFailed(string orderId, string exceptionMessage)
{
    failureCount.Add(1,
        new KeyValuePair<string, object?>("order_id", orderId),               // ❌ Unbounded
        new KeyValuePair<string, object?>("exception_message", exceptionMessage) // ❌ Unbounded
    );
}

Rules:

• Dimensions MUST be predefined at instrument creation
• Avoid dynamic/unbounded values (causes cardinality explosion: each unique value creates a new time series row)
• High-cardinality dimensions MUST be opt-in configuration
• Use low-cardinality identifiers: item type, queue name, outcome
• Consistent dimension names across components: myapp.region means same thing everywhere
• Avoid sensitive data
• Consider metric enrichment alternatives
• Users can enable metric exemplars for correlation (not through dimensions)

Performance Requirements

Instrumentation MUST be cheap by default. Follow these rules to minimize overhead:

Zero-Allocation Fast Path

// ✅ CORRECT: Guard with cheap checks
if (ActivitySource.HasListeners())
{
    using var activity = ActivitySource.StartActivity("Operation");
    // ... expensive work
}

// ✅ CORRECT: Use TagList (struct) for metrics
var tags = new TagList
{
    { "myapp.order_type", orderType },
    { "outcome", "success" }
};
counter.Add(1, tags);

Timing

// ✅ CORRECT: Timestamp math (no allocation)
var startTime = Stopwatch.GetTimestamp();
try
{
    await ProcessAsync();
}
finally
{
    var duration = Stopwatch.GetElapsedTime(startTime);
    metrics.OrderProcessingSucceeded(orderType, duration);
}

// ❌ WRONG: Allocates Stopwatch object
var stopwatch = Stopwatch.StartNew(); // ❌ Allocates

// ❌ WRONG: IDisposable timing class (allocates per use)
using (new MetricScope(metrics, "ProcessOrder")) // ❌ BAD
{
    ProcessOrder();
}

Avoid Hidden Allocations

// ❌ WRONG: String interpolation allocates
activity?.SetTag("item", $"Processing {itemId}"); // ❌ Allocates

// ✅ CORRECT: Check IsAllDataRequested first
if (activity?.IsAllDataRequested == true)
{
    activity.SetTag("item", $"Processing {itemId}");
}

// ❌ WRONG: LINQ allocates enumerators
activity?.SetTag("handlers", handlers.Select(h => h.Name).ToArray()); // ❌ Bad

// ✅ CORRECT: Manual construction or check first
if (activity?.IsAllDataRequested == true)
{
    activity.SetTag("handlers", string.Join(",", handlers.Select(h => h.Name)));
}

Rules:

• No Stopwatch.StartNew() (use timestamp math)
• No timing IDisposable wrappers as classes
• Prefer TagList (struct) over arrays/dictionaries
• No hidden work: avoid LINQ, string interpolation, async state machines in hot paths

Testing Requirements

Span Tests

[Test]
public async Task Should_create_processing_span_with_correct_parent()
{
    // Arrange
    using var parent = new Activity("Parent").Start();

    // Act
    await handler.Handle(item);

    // Assert
    var processingSpan = recordedActivities.Single(a => a.OperationName == "ProcessItem");
    Assert.AreEqual(parent.Id, processingSpan.ParentId);
    Assert.AreEqual("myapp.item_type", processingSpan.Tags.First().Key);
}

[Test]
public void Should_not_introduce_breaking_changes_to_span_names()
{
    // Ensures string values in span names are under test
    Assert.AreEqual("ProcessItem", MyFeature.SpanName);
}

Rules:

• Test which spans activities connect to
• Test string values (span names, tag names) to prevent breaking changes
• Remember: telemetry is part of public API

Versioning

• Telemetry versioning decoupled from package version
• Use SemVer semantics
• Traces and Metrics use separate versions (evolve independently)
• Start with pre-1.0.0 version until adoption/usefulness proven

private static readonly ActivitySource ActivitySource = new("MyApp.MyComponent", "0.9.0");
private readonly Meter meter = new("MyApp.MyComponent", "0.8.0");

References

• OpenTelemetry .NET Trace Documentation
• OpenTelemetry .NET Metrics Documentation
• OpenTelemetry Semantic Conventions
• Microsoft Distributed Tracing Instrumentation
• ASP.NET Core Metrics Examples
• OpenTelemetry Trace API Span Definition
• OpenTelemetry Exception Conventions
• OpenTelemetry Attribute Specification
• OpenTelemetry Cardinality Limits