Apply customizers to build when created embedded Mongo client

Closes gh-11132

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mongo/ReactiveMongoClientFactory.java

@@ -92,6 +92,7 @@ public class ReactiveMongoClientFactory {
 		ClusterSettings clusterSettings = ClusterSettings.builder()
 				.hosts(Collections.singletonList(new ServerAddress(host, port))).build();
 		builder.clusterSettings(clusterSettings);
+		customize(builder);
 		return MongoClients.create(builder.build());
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/mongo/ReactiveMongoClientFactoryTests.java

@@ -161,6 +161,16 @@ public class ReactiveMongoClientFactoryTests {
 		verify(customizer).customize(any(MongoClientSettings.Builder.class));
 	}
 
+	@Test
+	public void customizerIsInvokedForEmbeddedMongo() {
+		MongoProperties properties = new MongoProperties();
+		this.environment.setProperty("local.mongo.port", "27017");
+		MongoClientSettingsBuilderCustomizer customizer = mock(
+				MongoClientSettingsBuilderCustomizer.class);
+		createMongoClient(properties, this.environment, customizer);
+		verify(customizer).customize(any(MongoClientSettings.Builder.class));
+	}
+
 	private MongoClient createMongoClient(MongoProperties properties) {
 		return createMongoClient(properties, this.environment);
 	}