Add some constants used for parent/child process communication

Author: baronfel

src/Cli/Microsoft.DotNet.Cli.Utils/Activities.cs

@@ -6,15 +6,27 @@
 namespace Microsoft.DotNet.Cli.Utils;
 
 /// <summary>
-/// Contains helpers for working with <see cref="System.Diagnostics.Activity">Activities</see> in the .NET CLI.
+/// Contains helpers for working with <see cref="Activity">Activities</see> in the .NET CLI.
 /// </summary>
 public static class Activities
 {
-
     /// <summary>
     /// The main entrypoint for creating <see cref="Activity">Activities</see> in the .NET CLI.
     /// All activities created in the CLI should use this <see cref="ActivitySource"/>, to allow
     /// consumers to easily filter and trace CLI activities.
     /// </summary>
     public static ActivitySource Source { get; } = new("dotnet-cli", Product.Version);
+
+    /// <summary>
+    /// The environment variable used to transfer the chain of parent activity IDs.
+    /// This should be used when constructing new sub-processes in order to
+    /// track spans across calls.
+    /// </summary>
+    public const string TRACEPARENT = nameof(TRACEPARENT);
+    /// <summary>
+    /// The environment variable used to transfer the trace state of the parent activities.
+    /// This should be used when constructing new sub-processes in order to
+    /// track spans across calls.
+    /// </summary>
+    public const string TRACESTATE = nameof(TRACESTATE);
 }