[API] Support environment variable context propagation

OpenTelemetry.slnx

@@ -131,6 +131,7 @@
   <Folder Name="/examples/">
     <Project Path="examples/AspNetCore/Examples.AspNetCore.csproj" />
     <Project Path="examples/Console/Examples.Console.csproj" />
+    <Project Path="examples/EnvironmentVariables/Examples.EnvironmentVariables.csproj" />
     <Project Path="examples/FSharp/Examples.FSharp.fsproj" />
     <Project Path="examples/GrpcService/Examples.GrpcService.csproj" />
   </Folder>

examples/EnvironmentVariables/Examples.EnvironmentVariables.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>$(DefaultTargetFrameworkForExampleApps)</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="$(RepoRoot)\src\OpenTelemetry.Api\OpenTelemetry.Api.csproj" />
+  </ItemGroup>
+
+</Project>

examples/EnvironmentVariables/Program.cs

@@ -0,0 +1,235 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+using System.Collections;
+using System.Diagnostics;
+using System.Reflection;
+using System.Text;
+using OpenTelemetry;
+using OpenTelemetry.Context.Propagation;
+
+namespace Examples.EnvironmentVariables;
+
+internal static class Program
+{
+    private const string ActivitySourceName = "Examples.EnvironmentVariables";
+    private const string ChildModeArgument = "--child";
+
+    private static readonly ActivitySource ActivitySource = new(ActivitySourceName);
+    private static readonly TextMapPropagator Propagator = new CompositeTextMapPropagator(
+    [
+        new TraceContextPropagator(),
+        new BaggagePropagator(),
+    ]);
+
+    public static int Main(string[] args)
+    {
+        using var listener = CreateActivityListener();
+
+        return args.Contains(ChildModeArgument, StringComparer.Ordinal)
+            ? RunAsChild()
+            : RunAsParent();
+    }
+
+    private static int RunAsParent()
+    {
+        Baggage.ClearBaggage();
+        Baggage.SetBaggage("tenant.id", "contoso");
+        Baggage.SetBaggage("user.id", "alice");
+
+        using var activity = ActivitySource.StartActivity("parent-process");
+        if (activity == null)
+        {
+            Console.Error.WriteLine("Failed to create the parent activity.");
+            return 1;
+        }
+
+        WriteProcessContext("Parent", activity, default, Baggage.Current);
+
+        var startInfo = CreateChildStartInfo();
+
+        CopyCurrentEnvironment(startInfo.Environment);
+
+        var context = new PropagationContext(activity.Context, Baggage.Current);
+        Propagator.Inject(context, startInfo.Environment, EnvironmentVariableCarrier.Set);
+
+        Console.WriteLine("[Parent] Injected environment variables:");
+        WritePropagationFields("Parent", startInfo.Environment);
+        Console.WriteLine();
+
+        using var child = Process.Start(startInfo);
+        if (child == null)
+        {
+            Console.Error.WriteLine("Failed to start the child process.");
+            return 1;
+        }
+
+        var childStandardOutput = child.StandardOutput.ReadToEnd();
+        var childStandardError = child.StandardError.ReadToEnd();
+
+        child.WaitForExit();
+
+        if (!string.IsNullOrEmpty(childStandardOutput))
+        {
+            Console.WriteLine();
+            Console.Write(childStandardOutput);
+        }
+
+        if (!string.IsNullOrEmpty(childStandardError))
+        {
+            Console.Error.Write(childStandardError);
+        }
+
+        Console.WriteLine();
+        Console.WriteLine($"[Parent] Child process exited with code {child.ExitCode}.");
+
+        return child.ExitCode;
+    }
+
+    private static int RunAsChild()
+    {
+        var carrier = EnvironmentVariableCarrier.Capture();
+        var parentContext = Propagator.Extract(default, carrier, EnvironmentVariableCarrier.Get);
+
+        Baggage.Current = parentContext.Baggage;
+
+        using var activity = ActivitySource.StartActivity(
+            "child-process",
+            ActivityKind.Internal,
+            parentContext.ActivityContext);
+
+        if (activity == null)
+        {
+            Console.Error.WriteLine("Failed to create the child activity.");
+            return 1;
+        }
+
+        Console.WriteLine("  [Child] Captured propagated environment variables:");
+        WritePropagationFields("Child", carrier);
+        Console.WriteLine();
+
+        WriteProcessContext("Child", activity, parentContext.ActivityContext, Baggage.Current);
+
+        return 0;
+    }
+
+    private static ActivityListener CreateActivityListener()
+    {
+        var listener = new ActivityListener
+        {
+            ShouldListenTo = static source => source.Name == ActivitySourceName,
+            Sample = static (ref _) => ActivitySamplingResult.AllDataAndRecorded,
+            SampleUsingParentId = static (ref _) => ActivitySamplingResult.AllDataAndRecorded,
+        };
+
+        ActivitySource.AddActivityListener(listener);
+
+        return listener;
+    }
+
+    private static ProcessStartInfo CreateChildStartInfo()
+    {
+        var processPath = Environment.ProcessPath
+            ?? throw new InvalidOperationException("The current process path is unavailable.");
+
+        var entryAssemblyPath = Assembly.GetEntryAssembly()?.Location
+            ?? throw new InvalidOperationException("The entry assembly path is unavailable.");
+
+        var fileName = Path.GetFileNameWithoutExtension(processPath);
+
+        var startInfo = new ProcessStartInfo(processPath)
+        {
+            RedirectStandardError = true,
+            RedirectStandardOutput = true,
+            UseShellExecute = false,
+        };
+
+        if (string.Equals(fileName, "dotnet", StringComparison.OrdinalIgnoreCase))
+        {
+            startInfo.ArgumentList.Add(entryAssemblyPath);
+        }
+
+        startInfo.ArgumentList.Add(ChildModeArgument);
+
+        return startInfo;
+    }
+
+    private static void CopyCurrentEnvironment(IDictionary<string, string?> environment)
+    {
+        foreach (DictionaryEntry variable in Environment.GetEnvironmentVariables())
+        {
+            environment[(string)variable.Key] = variable.Value?.ToString();
+        }
+    }
+
+    private static void WriteProcessContext(
+        string role,
+        Activity activity,
+        ActivityContext parentContext,
+        Baggage baggage)
+    {
+        string indent = role is "Parent" ? string.Empty : "  ";
+
+        Console.WriteLine($"{indent}[{role}] ProcessId: {Environment.ProcessId}");
+        Console.WriteLine($"{indent}[{role}] TraceId: {activity.TraceId}");
+        Console.WriteLine($"{indent}[{role}] SpanId: {activity.SpanId}");
+        Console.WriteLine($"{indent}[{role}] ParentSpanId: {FormatSpanId(activity.ParentSpanId)}");
+
+        if (parentContext != default)
+        {
+            Console.WriteLine($"{indent}[{role}] ExtractedParentTraceId: {parentContext.TraceId}");
+            Console.WriteLine($"{indent}[{role}] ExtractedParentSpanId: {FormatSpanId(parentContext.SpanId)}");
+        }
+
+        Console.WriteLine($"{indent}[{role}] Baggage: {FormatBaggage(baggage)}");
+
+        static string FormatSpanId(ActivitySpanId spanId)
+        {
+            return spanId == default ? "<none>" : spanId.ToString();
+        }
+
+        static string FormatBaggage(Baggage baggage)
+        {
+            if (baggage.Count == 0)
+            {
+                return "<empty>";
+            }
+
+            var builder = new StringBuilder();
+
+            foreach (var item in baggage.GetBaggage())
+            {
+                if (builder.Length > 0)
+                {
+                    builder.Append(", ");
+                }
+
+                builder.Append(item.Key);
+                builder.Append('=');
+                builder.Append(item.Value);
+            }
+
+            return builder.ToString();
+        }
+    }
+
+    private static void WritePropagationFields<T>(string role, T carrier)
+        where T : IEnumerable<KeyValuePair<string, string?>>
+    {
+        if (Propagator.Fields is not { Count: > 0 } fields)
+        {
+            return;
+        }
+
+        string indent = role is "Parent" ? string.Empty : "  ";
+
+        foreach (var field in fields)
+        {
+            var normalized = EnvironmentVariableCarrier.NormalizeKey(field);
+            var values = EnvironmentVariableCarrier.Get(carrier, field);
+            var value = values?.FirstOrDefault();
+
+            Console.WriteLine($"{indent}[{role}]   {normalized}={value ?? "<not set>"}");
+        }
+    }
+}

src/OpenTelemetry.Api/.publicApi/Stable/PublicAPI.Unshipped.txt

@@ -0,0 +1,6 @@
+OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier
+static OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier.Capture() -> System.Collections.Generic.IReadOnlyDictionary<string!, string?>!
+static OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier.Capture(System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string!, string?>>! environmentVariables) -> System.Collections.Generic.IReadOnlyDictionary<string!, string?>!
+static OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier.Get<T>(T carrier, string! key) -> System.Collections.Generic.IEnumerable<string!>?
+static OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier.NormalizeKey(string! key) -> string!
+static OpenTelemetry.Context.Propagation.EnvironmentVariableCarrier.Set<T>(T carrier, string! key, string! value) -> void