Add Program Context to UserAgent Header (#517)

Add Program context comment to user agent header to enable tracking of
usage across our credprovider wrappers (ex conda, artifacts-keyring)


example header with changes:
`(NuGet) CredentialProvider.Microsoft/1.2.1 (Windows; X64; Microsoft
Windows 10.0.22631) CLR/6.0.33 (.NETCoreApp,Version=v6.0; win10-x64;
.NET 6.0.33)`

before:
`CredentialProvider.Microsoft/1.2.1 (Windows; X64; Microsoft Windows
10.0.22631) CLR/6.0.33 (.NETCoreApp,Version=v6.0; win10-x64; .NET
6.0.33)`

Author: embetten

.vscode/launch.json

@@ -2,7 +2,7 @@
     "version": "0.2.0",
     "configurations": [
         {
-            "name": ".NET Core Launch (console)",
+            "name": "net 6 launch",
             "type": "coreclr",
             "request": "launch",
             "preLaunchTask": "build",
@@ -13,7 +13,10 @@
             ],
             "cwd": "${workspaceFolder}/CredentialProvider.Microsoft",
             "console": "integratedTerminal",
-            "stopAtEntry": false
+            "stopAtEntry": false,
+            "env": {
+                "ARTIFACTS_CREDENTIALPROVIDER_PROGRAM_CONTEXT":"nuGET"
+            }
         },
         {
             "name": ".NET Core Attach",

CredentialProvider.Microsoft/Program.cs

@@ -113,6 +113,7 @@ public static async Task<int> Main(string[] args)
                             multiLogger.Add(new PluginConnectionLogger(plugin.Connection));
                             multiLogger.Verbose(Resources.RunningInPlugin);
                             multiLogger.Verbose(string.Format(Resources.CommandLineArgs, PlatformInformation.GetProgramVersion(), Environment.CommandLine));
+                            EnvUtil.SetProgramContextInEnvironment(Context.NuGet);
 
                             await WaitForPluginExitAsync(plugin, multiLogger, TimeSpan.FromMinutes(2)).ConfigureAwait(continueOnCapturedContext: false);
                         }

CredentialProvider.Microsoft/Util/EnvUtil.cs

@@ -39,6 +39,8 @@ public static class EnvUtil
         public const string EndpointCredentials = "ARTIFACTS_CREDENTIALPROVIDER_FEED_ENDPOINTS";
         public const string BuildTaskExternalEndpoints = "VSS_NUGET_EXTERNAL_FEED_ENDPOINTS";
 
+        public const string ProgramContext = "ARTIFACTS_CREDENTIALPROVIDER_PROGRAM_CONTEXT";
+
         public static bool GetLogPIIEnabled()
         {
             return GetEnabledFromEnvironment(LogPIIEnvVar, defaultValue: false);
@@ -182,6 +184,24 @@ public static int GetDeviceFlowTimeoutFromEnvironmentInSeconds(ILogger logger)
             return null;
         }
 
+        public static Context? GetProgramContextFromEnvironment()
+        {
+            var context = Environment.GetEnvironmentVariable(ProgramContext);
+
+            if (!string.IsNullOrWhiteSpace(context) && Enum.TryParse<Context>(context, ignoreCase: true, out Context result))
+            {
+                return result;
+            }
+
+            return null;
+        }
+
+        public static void SetProgramContextInEnvironment(Context context)
+        {
+            Environment.SetEnvironmentVariable(ProgramContext, context.ToString());
+            return;
+        }
+
         private static bool GetEnabledFromEnvironment(string envVar, bool defaultValue = true)
         {
             if (bool.TryParse(Environment.GetEnvironmentVariable(envVar), out bool result))

CredentialProvider.Microsoft/Util/HttpClientFactory.cs

@@ -3,6 +3,7 @@
 // Licensed under the MIT license.
 
 using System.Net.Http;
+using System.Net.Http.Headers;
 using Microsoft.Artifacts.Authentication;
 
 namespace NuGetCredentialProvider.Util
@@ -26,7 +27,32 @@ static HttpClientFactory()
                 UseDefaultCredentials = true
             });
 
+            // Add program context to headers if available
+            if (ProgramContext != null) 
+            {
+                httpClient.DefaultRequestHeaders.UserAgent.Add(ProgramContext);
+            }
+
             httpClientFactory = new(httpClient);
         }
+
+        private static ProductInfoHeaderValue ProgramContext
+        {
+            get
+            {
+                var context = EnvUtil.GetProgramContextFromEnvironment();
+                return context != null 
+                    ? new ProductInfoHeaderValue($"({context})")
+                    : null; 
+            }
+        }
+    }
+
+    public enum Context
+    {
+        Maven,
+        NuGet,
+        Pip,
+        Conda, 
     }
 }