Add internal for setting caller as compat cmdlet (#1432)

Author: adityapatwardhan

src/code/InternalHooks.cs

@@ -0,0 +1,18 @@
+// Copyright (c) Microsoft Corporation. All rights reserved.
+// Licensed under the MIT License.
+
+using System.Reflection;
+
+namespace Microsoft.PowerShell.PSResourceGet.UtilClasses
+{
+    public class InternalHooks
+    {
+        internal static bool InvokedFromCompat;
+
+        public static void SetTestHook(string property, object value)
+        {
+            var fieldInfo = typeof(InternalHooks).GetField(property, BindingFlags.Static | BindingFlags.NonPublic);
+            fieldInfo?.SetValue(null, value);
+        }
+    }
+}

src/code/ServerFactory.cs

@@ -25,7 +25,11 @@ static UserAgentInfo()
         private static string _psResourceGetVersion;
         private static string _distributionChannel;
 
-        internal static string UserAgentString => $"PSResourceGet/{_psResourceGetVersion} PowerShell/{_psVersion} DistributionChannel/{_distributionChannel}";
+        internal static string UserAgentString()
+        {
+            string psGetCompat = InternalHooks.InvokedFromCompat ? "true" : "false";
+            return $"PSResourceGet/{_psResourceGetVersion} PowerShell/{_psVersion} DistributionChannel/{_distributionChannel} PowerShellGetCompat/{psGetCompat}";
+        }
     }
 
     internal class ServerFactory
@@ -34,7 +38,7 @@ public static ServerApiCall GetServer(PSRepositoryInfo repository, PSCmdlet cmdl
         {
             PSRepositoryInfo.APIVersion repoApiVersion = repository.ApiVersion;
             ServerApiCall currentServer = null;
-            string userAgentString = UserAgentInfo.UserAgentString;
+            string userAgentString = UserAgentInfo.UserAgentString();
 
             switch (repoApiVersion)
             {