Add support for registering service definition bean

spring-grpc-docs/src/main/antora/modules/ROOT/pages/server.adoc

@@ -5,12 +5,15 @@ This section describes core concepts that Spring gRPC uses on the server side. W
 You only need to provide one or more beans of type `BindableService` to create a gRPC server, provided the classpath contains an implementation of a `Server`. The `BindableService` is a gRPC service that can be bound to a server.
 The `Server` is the gRPC server that listens for incoming requests and routes them to the appropriate service implementation.
 
-== Create a `BindableService`
+== Create a gRPC Service
 
-To create a gRPC server, you need to provide one or more beans of type `BindableService`.
+To create a gRPC server, you need to provide one or more beans of type `BindableService` or `ServerServiceDefinition`.
 There are some `BindableServices` available off the shelf that you could include in your application (an example is the reflection service from the `grpc-services` artifact which allows clients to browse the metadata of your services and download the Portobuf files). 
 Very commonly, you will create your own `BindableService` by extending the generated service implementation from your Protobuf file.
-The easiest way to do it is to simply add a Spring `@Service` annotation to the implementation class and have it picked up by the `@ComponentScan` in your Spring Boot application.
+The easiest way to activate it is to simply add a Spring `@Service` annotation to the implementation class and have it picked up by the `@ComponentScan` in your Spring Boot application.
+Alternatively, you could create an instance of it in a `@Configuration` class and return it as a bean, or return it's `ServerServiceDefinition`.
+The last option is useful if you need to add specific behaviour, like an interceptor, to the service.
+It is an error to have more than one instance of the same service name, so don't create both a `BindableService` and a `ServerServiceDefinition` bean for the same service.
 
 == Netty Server
 

spring-grpc-spring-boot-autoconfigure/src/main/java/org/springframework/grpc/autoconfigure/server/DefaultGrpcServiceDiscoverer.java

@@ -16,6 +16,7 @@
 
 package org.springframework.grpc.autoconfigure.server;
 
+import java.util.ArrayList;
 import java.util.List;
 
 import io.grpc.BindableService;
@@ -35,13 +36,20 @@ class DefaultGrpcServiceDiscoverer implements GrpcServiceDiscoverer {
 
 	private final ObjectProvider<BindableService> grpcServicesProvider;
 
-	DefaultGrpcServiceDiscoverer(ObjectProvider<BindableService> grpcServicesProvider) {
+	private ObjectProvider<ServerServiceDefinition> grpcServiceDefinitionsProvider;
+
+	DefaultGrpcServiceDiscoverer(ObjectProvider<BindableService> grpcServicesProvider,
+			ObjectProvider<ServerServiceDefinition> grpcServiceDefinitionsProvider) {
 		this.grpcServicesProvider = grpcServicesProvider;
+		this.grpcServiceDefinitionsProvider = grpcServiceDefinitionsProvider;
 	}
 
 	@Override
 	public List<ServerServiceDefinition> findServices() {
-		return grpcServicesProvider.orderedStream().map(BindableService::bindService).toList();
+		List<ServerServiceDefinition> list = new ArrayList<>(
+				grpcServicesProvider.orderedStream().map(BindableService::bindService).toList());
+		list.addAll(grpcServiceDefinitionsProvider.orderedStream().toList());
+		return list;
 	}
 
 }

spring-grpc-spring-boot-autoconfigure/src/main/java/org/springframework/grpc/autoconfigure/server/GrpcServerAutoConfiguration.java

@@ -36,6 +36,7 @@
 import io.grpc.CompressorRegistry;
 import io.grpc.DecompressorRegistry;
 import io.grpc.ServerBuilder;
+import io.grpc.ServerServiceDefinition;
 
 /**
  * {@link EnableAutoConfiguration Auto-configuration} for gRPC server-side components.
@@ -75,8 +76,9 @@ ServerBuilderCustomizers serverBuilderCustomizers(ObjectProvider<ServerBuilderCu
 
 	@ConditionalOnMissingBean
 	@Bean
-	GrpcServiceDiscoverer grpcServiceDiscoverer(ObjectProvider<BindableService> bindableServicesProvider) {
-		return new DefaultGrpcServiceDiscoverer(bindableServicesProvider);
+	GrpcServiceDiscoverer grpcServiceDiscoverer(ObjectProvider<BindableService> bindableServicesProvider,
+			ObjectProvider<ServerServiceDefinition> serviceDefinitionsProvider) {
+		return new DefaultGrpcServiceDiscoverer(bindableServicesProvider, serviceDefinitionsProvider);
 	}
 
 	@ConditionalOnBean(CompressorRegistry.class)

spring-grpc-spring-boot-autoconfigure/src/test/java/org/springframework/grpc/autoconfigure/server/GrpcServerAutoConfigurationTests.java

@@ -134,6 +134,17 @@ void grpcServiceDiscovererAutoConfiguredAsExpected() {
 				.containsExactly(this.serviceDefinition));
 	}
 
+	@Test
+	void grpcServiceDiscovererWithServiceDefinitionAsExpected() {
+		ServerServiceDefinition serviceDefinition = ServerServiceDefinition.builder("my-other-service").build();
+		this.contextRunnerWithLifecyle()
+			.withBean(ServerServiceDefinition.class, () -> serviceDefinition)
+			.run((context) -> assertThat(context).getBean(GrpcServiceDiscoverer.class)
+				.extracting(GrpcServiceDiscoverer::findServices,
+						InstanceOfAssertFactories.list(ServerServiceDefinition.class))
+				.containsExactly(this.serviceDefinition, serviceDefinition));
+	}
+
 	@Test
 	void whenHasUserDefinedServerBuilderCustomizersDoesNotAutoConfigureBean() {
 		ServerBuilderCustomizers customCustomizers = mock(ServerBuilderCustomizers.class);