Merge pull request #1702 from jmcdo29/docs/custom-proxy

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

Author: kamilmysliwiec

content/microservices/custom-transport.md

@@ -215,3 +215,36 @@ And that's what you should see in the console:
 connect
 event to dispatch:  { pattern: 'event', data: 'Hello world!' }
 ```
+
+#### Message serialization
+
+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`.
+
+```typescript
+@@filename(error-handling.proxy)
+import { ClientTcp, RpcException } from '@nestjs/microservices';
+
+class ErrorHandlingProxy extends ClientTCP {
+  serializeError(err: Error) {
+    return new RpcException(err);
+  }
+}
+```
+
+and then use it in the `ClientsModule` like so:
+
+```typescript
+@@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`.
+>