Structured logging, metrics and traces

#:package Microsoft.Extensions.Hosting@10.0.12
#:property PublishAot=false
// Three orders, three signals: a structured log line per order, two metric instruments, and a trace of each order
// with a child for its payment. A MeterListener and an ActivityListener in the same program keep what an exporter
// would receive, and print it once the logger has written its lines.
using System.Diagnostics;
using System.Diagnostics.Metrics;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

List<string> measurements = [], spans = [];
using MeterListener meters = new() { InstrumentPublished = (instrument, l) => { if (instrument.Meter.Name == "Shop.Orders") l.EnableMeasurementEvents(instrument); } };
meters.SetMeasurementEventCallback<long>((i, value, tags, _) => measurements.Add($"{i.Name} {value} {Tags(tags)}"));
meters.SetMeasurementEventCallback<double>((i, value, tags, _) => measurements.Add($"{i.Name} {value} {Tags(tags)}"));
meters.Start();
using ActivityListener tracer = new()
{
    ShouldListenTo = source => source.Name == "Shop.Orders",
    Sample = (ref ActivityCreationOptions<ActivityContext> _) => ActivitySamplingResult.AllDataAndRecorded,
    ActivityStopped = a => spans.Add($"{(a.Parent is null ? "" : "  ")}{a.DisplayName} {string.Join(", ", a.Tags.Select(t => $"{t.Key}={t.Value}"))} status={a.Status}"),
};
ActivitySource.AddActivityListener(tracer);

HostApplicationBuilder builder = Host.CreateApplicationBuilder(args);
builder.Logging.ClearProviders();
builder.Logging.AddJsonConsole();
builder.Services.AddSingleton<OrderProcessor>();
using (IHost host = builder.Build())
{
    OrderProcessor processor = host.Services.GetRequiredService<OrderProcessor>();
    processor.Process(new Order("A-1001", 499m, "card"));
    processor.Process(new Order("A-1002", 1250m, "wallet"));
    processor.Process(new Order("A-1003", 80m, "card", Declined: true));
}   // disposing the host flushes the console logger, so its lines come first
Console.WriteLine("metrics:");
measurements.ForEach(m => Console.WriteLine($"  {m}"));
Console.WriteLine("traces:");
spans.ForEach(s => Console.WriteLine($"  {s}"));

static string Tags(ReadOnlySpan<KeyValuePair<string, object?>> tags)
{
    List<string> parts = [];
    foreach (var tag in tags) parts.Add($"{tag.Key}={tag.Value}");
    return parts.Count == 0 ? "" : "{" + string.Join(", ", parts) + "}";
}

record Order(string Id, decimal Amount, string Method, bool Declined = false);

sealed class OrderProcessor(ILogger<OrderProcessor> logger)
{
    static readonly ActivitySource Source = new("Shop.Orders");
    static readonly Meter Meter = new("Shop.Orders");
    static readonly Counter<long> Processed = Meter.CreateCounter<long>("shop.orders.processed", "{order}");
    static readonly Histogram<double> Amounts = Meter.CreateHistogram<double>("shop.orders.amount", "INR");

    public void Process(Order order)
    {
        using Activity? activity = Source.StartActivity("ProcessOrder");
        activity?.SetTag("order.id", order.Id);
        using (Activity? payment = Source.StartActivity("ChargePayment"))
        {
            payment?.SetTag("payment.method", order.Method);
            if (order.Declined)
                payment?.SetStatus(ActivityStatusCode.Error, "declined");
        }
        string status = order.Declined ? "declined" : "paid";
        activity?.SetStatus(order.Declined ? ActivityStatusCode.Error : ActivityStatusCode.Ok);
        logger.LogInformation("Order {OrderId} {Status}: {Amount} by {Method}", order.Id, status, order.Amount, order.Method);
        Processed.Add(1, new KeyValuePair<string, object?>("status", status));
        Amounts.Record((double)order.Amount);
    }
}

from session import run

# network: true. signals.cs names Microsoft.Extensions.Hosting 10.0.12 in its #:package line, and dotnet run
# restores it from nuget.org into the session's own, empty home folder.
run("""
cd signals
dotnet run signals.cs
""", network=True)

$ cd signals
$ dotnet run signals.cs
{"EventId":0,"LogLevel":"Information","Category":"OrderProcessor","Message":"Order A-1001 paid: 499 by card","State":{"OrderId":"A-1001","Status":"paid","Amount":499,"Method":"card","{OriginalFormat}":"Order {OrderId} {Status}: {Amount} by {Method}"}}
{"EventId":0,"LogLevel":"Information","Category":"OrderProcessor","Message":"Order A-1002 paid: 1250 by wallet","State":{"OrderId":"A-1002","Status":"paid","Amount":1250,"Method":"wallet","{OriginalFormat}":"Order {OrderId} {Status}: {Amount} by {Method}"}}
{"EventId":0,"LogLevel":"Information","Category":"OrderProcessor","Message":"Order A-1003 declined: 80 by card","State":{"OrderId":"A-1003","Status":"declined","Amount":80,"Method":"card","{OriginalFormat}":"Order {OrderId} {Status}: {Amount} by {Method}"}}
metrics:
  shop.orders.processed 1 {status=paid}
  shop.orders.amount 499
  shop.orders.processed 1 {status=paid}
  shop.orders.amount 1250
  shop.orders.processed 1 {status=declined}
  shop.orders.amount 80
traces:
    ChargePayment payment.method=card status=Unset
  ProcessOrder order.id=A-1001 status=Ok
    ChargePayment payment.method=wallet status=Unset
  ProcessOrder order.id=A-1002 status=Ok
    ChargePayment payment.method=card status=Error
  ProcessOrder order.id=A-1003 status=Error