feat: use getMetadataByDecorator of discover service instead of SetMetadata

Author: vahidvdn

content/fundamentals/discovery-service.md

@@ -77,19 +77,19 @@ for (const provider of providers) {
 Suppose you have a custom decorator that adds metadata to a provider:
 
 ```typescript
-import { SetMetadata } from '@nestjs/common';
+import { DiscoveryService } from '@nestjs/core';
 
-export const CustomMetadata = (value: string) => SetMetadata('custom:metadataKey', value);
+export const Pets = DiscoveryService.createDecorator();
 ```
 
 And you use it in a service:
 
 ```typescript
 import { Injectable } from '@nestjs/common';
-import { CustomMetadata } from './custom-metadata.decorator';
+import { Pets } from './custom-metadata.decorator';
 
 @Injectable()
-@CustomMetadata('example-value')
+@Pets('cats')
 export class CustomService {}
 ```
 
@@ -98,12 +98,11 @@ Now, you can use `DiscoveryService` to find all providers with this metadata:
 ```typescript
 const providers = this.discoveryService.getProviders();
 
-const filteredProviders = providers.filter((provider) => {
-  if (!provider.instance) return null;
-  return !!this.reflector.get(metadataPathToken, provider.instance.constructor);
-});
+const [provider] = providers.filter(
+  (prov) => this.discoveryService.getMetadataByDecorator(Pets, prov) === 'cats',
+);
 
-console.log('Providers with custom metadata:', filteredProviders);
+console.log('Providers with cats metadata:', provider);
 ```
 
 ### Conclusion