Update migration guide (Azure#22797)

* Update migration guide

* Fix snippet

Author: JoshLove-msft

sdk/servicebus/Azure.Messaging.ServiceBus/MigrationGuide.md

@@ -440,8 +440,7 @@ using (var ts = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
 
 There are a few features that are yet to be implemented in `Azure.Messaging.ServiceBus`, but were present in the previous library `Microsoft.Azure.ServiceBus`. The plan is to add these features in upcoming releases (unless otherwise noted), but they will not be available in the version 7.0.0:
 
--   **Plugins** - In the previous library, Microsoft.Azure.ServiceBus, users could [register plugins](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Microsoft.Azure.ServiceBus/src/QueueClient.cs#L527) that would alter an outgoing message before serialization, or alter an incoming message after being deserialized. These extension points allowed users of the Service Bus library to use common OSS extensions to enhance their applications without having to implement their own logic, and without having to wait for the SDK to explicitly support the needed feature. For instance, one use of the plugin functionality is to implement the [claim-check pattern](https://www.nuget.org/packages/ServiceBus.AttachmentPlugin/) to send and receive messages that exceed the Service Bus message size limits. This feature is not yet supported in the new library but will be added in an upcoming release. Support for this feature can be tracked via https://github.com/Azure/azure-sdk-for-net/issues/12943.
-
+-   **Plugins** - In the previous library, Microsoft.Azure.ServiceBus, users could [register plugins](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Microsoft.Azure.ServiceBus/src/QueueClient.cs#L527) that would alter an outgoing message before serialization, or alter an incoming message after being deserialized. These extension points allowed users of the Service Bus library to use common OSS extensions to enhance their applications without having to implement their own logic, and without having to wait for the SDK to explicitly support the needed feature. For instance, one use of the plugin functionality is to implement the [claim-check pattern](https://www.nuget.org/packages/ServiceBus.AttachmentPlugin/) to send and receive messages that exceed the Service Bus message size limits. To achieve similar functionality with `Azure.Messaging.ServiceBus`, you can extend the various types as demonstrated in the [extensibility sample](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample09_Extensibility.md).
 ## Additional samples
 
 More examples can be found at:

sdk/servicebus/Azure.Messaging.ServiceBus/samples/README.md

@@ -19,3 +19,4 @@ description: Samples for the Azure.Messaging.ServiceBus client library
 - [Working with Transactions](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample06_Transactions.md)
 - [CRUD Operations](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample07_CrudOperations.md)
 - [Interop](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample08_Interop.md)
+- [Extensibility](https://github.com/Azure/azure-sdk-for-net/blob/main/sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample09_Extensibility.md)

sdk/servicebus/Azure.Messaging.ServiceBus/samples/Sample09_Extensibility.md

@@ -90,64 +90,38 @@ public class PluginProcessor : ServiceBusProcessor
 ```
 
 And a `PluginSessionProcessor`:
-```C# Snippet:End2EndPluginSessionProcessor
-string connectionString = "<connection_string>";
-string queueName = "<queue_name>";
-// since ServiceBusClient implements IAsyncDisposable we create it with "await using"
-await using var client = new ServiceBusClient(connectionString);
-await using ServiceBusSender sender = client.CreatePluginSender(queueName, new List<Func<ServiceBusMessage, Task>>()
+```C# Snippet:PluginSessionProcessor
+public class PluginSessionProcessor : ServiceBusSessionProcessor
 {
-    message =>
-    {
-        message.Subject = "Updated subject";
-        message.SessionId = "sessionId";
-        Console.WriteLine("First send plugin executed!");
-    return Task.CompletedTask;
-    },
-    message =>
-    {
-        Console.WriteLine(message.Subject); // prints "Updated subject"
-        Console.WriteLine(message.SessionId); // prints "sessionId"
-        Console.WriteLine("Second send plugin executed!");
-        return Task.CompletedTask;
-    },
-});
-
-await sender.SendMessageAsync(new ServiceBusMessage(Encoding.UTF8.GetBytes("First")));
+    private IEnumerable<Func<ServiceBusReceivedMessage, Task>> _plugins;
 
-await using ServiceBusSessionProcessor processor = client.CreatePluginSessionProcessor(queueName, new List<Func<ServiceBusReceivedMessage, Task>>()
-{
-    message =>
+    internal PluginSessionProcessor(string queueName, ServiceBusClient client, IEnumerable<Func<ServiceBusReceivedMessage, Task>> plugins, ServiceBusSessionProcessorOptions options) :
+        base(client, queueName, options)
     {
-        var rawMessage = message.GetRawAmqpMessage();
-        rawMessage.Properties.Subject = "Received subject";
-        Console.WriteLine("First receive plugin executed!");
-        return Task.CompletedTask;
-    },
-    message =>
+        _plugins = plugins;
+    }
+
+    internal PluginSessionProcessor(string topicName, string subscriptionName, ServiceBusClient client, IEnumerable<Func<ServiceBusReceivedMessage, Task>> plugins, ServiceBusSessionProcessorOptions options) :
+        base(client, topicName, subscriptionName, options)
     {
-        Console.WriteLine(message.Subject); // prints "Received subject"
-        var rawMessage = message.GetRawAmqpMessage();
-        rawMessage.Properties.Subject = "Last subject";
-        Console.WriteLine("Second receive plugin executed!");
-        return Task.CompletedTask;
-    },
-});
+        _plugins = plugins;
+    }
 
-processor.ProcessMessageAsync += args =>
-{
-    Console.WriteLine(args.Message.Subject);
-    return Task.CompletedTask;
-};
+    protected internal override async Task OnProcessSessionMessageAsync(ProcessSessionMessageEventArgs args)
+    {
+        foreach (var plugin in _plugins)
+        {
+            await plugin.Invoke(args.Message);
+        }
 
-processor.ProcessErrorAsync += args =>
-{
-    Console.WriteLine(args.Exception);
-    return Task.CompletedTask;
-};
+        await base.OnProcessSessionMessageAsync(args);
+    }
 
-await processor.StartProcessingAsync();
-Console.ReadKey();
+    protected internal override Task OnProcessErrorAsync(ProcessErrorEventArgs args)
+    {
+        return Task.CompletedTask;
+    }
+}
 ```
 
 ### Defining extension methods

sdk/servicebus/Azure.Messaging.ServiceBus/tests/Samples/Plugins/PluginSessionProcessor.cs

@@ -7,6 +7,7 @@
 
 namespace Azure.Messaging.ServiceBus.Tests.Samples
 {
+    #region Snippet:PluginSessionProcessor
     public class PluginSessionProcessor : ServiceBusSessionProcessor
     {
         private IEnumerable<Func<ServiceBusReceivedMessage, Task>> _plugins;
@@ -38,4 +39,5 @@ protected internal override Task OnProcessErrorAsync(ProcessErrorEventArgs args)
             return Task.CompletedTask;
         }
     }
+    #endregion
 }