Distributed Tracing with OpenTelemetry

using System.Diagnostics;

var source = new ActivitySource("Shop.Inventory");

// Nobody is listening yet: no allocation, no span.
using (var first = source.StartActivity("inventory.reserve"))
    Console.WriteLine($"without a listener: {(first is null ? "null" : "created")}");

using var listener = new ActivityListener
{
    ShouldListenTo = s => s.Name == "Shop.Inventory",
    Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllDataAndRecorded,
    ActivityStopped = a => Console.WriteLine(
        $"stopped {a.DisplayName} trace={a.TraceId} span={a.SpanId} parent={a.ParentSpanId} {a.Duration.TotalMilliseconds:F0} ms"),
};
ActivitySource.AddActivityListener(listener);

using (var parent = source.StartActivity("checkout.place_order"))
{
    parent?.SetTag("order.id", 10482);
    using var child = source.StartActivity("inventory.reserve");
    child?.SetTag("product.sku", "SKU-1042");
    await Task.Delay(20);
}

apiVersion: apps/v1
kind: Deployment
metadata:
  name: orders-api
spec:
  replicas: 3
  selector:
    matchLabels:
      app: orders-api
  template:
    metadata:
      labels:
        app: orders-api
    spec:
      containers:
        - name: orders-api
          image: registry.example.com/shop/orders-api:1.4.2
          env:
            - name: OTEL_SERVICE_NAME
              value: orders-api
            - name: OTEL_EXPORTER_OTLP_ENDPOINT
              value: http://otel-collector.observability:4317
            - name: OTEL_EXPORTER_OTLP_PROTOCOL
              value: grpc
            - name: POD_NAME
              valueFrom:
                fieldRef:
                  fieldPath: metadata.name
            - name: OTEL_RESOURCE_ATTRIBUTES
              value: deployment.environment.name=production,k8s.pod.name=$(POD_NAME)

using System.Diagnostics;

public sealed record OrderPlacedMessage(int OrderId, decimal Total, Dictionary<string, string> Headers);

public static class OrderMessaging
{
    public static readonly ActivitySource Source = new("Shop.Messaging");

    public static async Task PublishAsync(int orderId, decimal total, Func<OrderPlacedMessage, Task> send)
    {
        var headers = new Dictionary<string, string>();
        // The producer span wraps the broker call, so it records the publish latency and failures.
        using var activity = Source.StartActivity("orders publish", ActivityKind.Producer);
        activity?.SetTag("messaging.destination.name", "orders");
        // Writes traceparent (and tracestate, baggage) for whichever activity is current.
        DistributedContextPropagator.Current.Inject(activity ?? Activity.Current, headers,
            static (carrier, key, value) => ((Dictionary<string, string>)carrier!)[key] = value);
        await send(new OrderPlacedMessage(orderId, total, headers));
    }

    public static async Task HandleAsync(OrderPlacedMessage message, Func<int, Task> process)
    {
        DistributedContextPropagator.Current.ExtractTraceIdAndState(message.Headers,
            static (object? carrier, string name, out string? value, out IEnumerable<string>? values) =>
            {
                values = null;
                ((Dictionary<string, string>)carrier!).TryGetValue(name, out value);
            },
            out var traceParent, out var traceState);

        ActivityContext.TryParse(traceParent, traceState, out var parent);
        using var activity = Source.StartActivity("orders process", ActivityKind.Consumer, parent);
        activity?.SetTag("messaging.destination.name", "orders");
        activity?.SetTag("order.id", message.OrderId);
        await process(message.OrderId);
    }
}