docs(microservice): add documentation for new custom client proxy

Author: jmcdo29

content/microservices/basics.md

@@ -280,6 +280,36 @@ async onApplicationBootstrap() {
 
 If the connection cannot be created, the `connect()` method will reject with the corresponding error object.
 
+#### Extending the ClientProxy
+
+If you need to add some custom logic around the serialization of responses on the client side, you can use a custom class that extends the `ClientProxy` class or one of its child classes. For modifying successful requests you can override the `serializeResponse` method, and for modifying any errors that go through this client you can override the `serializeError` method. To make use of this custom class, you can pass the class itself to the `ClientsModule.register()` method using the `customClass` property. Below is an example of a custom `ClientProxy` that serializes each error into an `RpcException`.
+
+```ts
+@@filename(error-handling.proxy)
+import { ClientTcp, RpcException } from '@nestjs/microservices';
+
+class ErrorHandlingProxy extends ClientTCP {
+  serializeError(err) {
+    return new RpcException(err);
+  }
+}
+```
+
+```ts
+@@filename(app.module)
+@Module({
+  imports: [
+    ClientsModule.register({
+      name: 'CustomProxy',
+      customClass: ErrorHandlingProxy,
+    }),
+  ]
+})
+export class AppModule
+```
+
+> info **hint** This is the class itself being passed to `customClass`, not an instance of the class. Nest will create the instance under the hood for you, and will pass any options given to the `options` property to the new `ClientProxy`.
+
 #### Sending messages
 
 The `ClientProxy` exposes a `send()` method. This method is intended to call the microservice and returns an `Observable` with its response. Thus, we can subscribe to the emitted values easily.