Add support for Kotlin Coroutine based client stubs

[imberda]

The problem

This project already supports the creation of different GRPC stub types (Blocking, Future, etc.). However GRPC Kotlin is and will be increasingly popular. When GRPC code is generated with grpc-kotlin the client stub extends AbstractCoroutineStub. It's currently not possible to instantiate instances of these stubs with this project.

The solution

The suggested solution is:

1. Create an optional dependency on io.grpc:grpc-kotlin-stub:0.2.0

2. Create a new stub factory - something like

   public class CoroutineStubFactory implements StubFactory {
   
       @Override
       public AbstractStub<?> createStub(Class<? extends AbstractStub<?>> stubType, Channel channel) {
   
           try {
               final Constructor<? extends AbstractStub<?>> constructor = stubType.getConstructor(Channel.class, CallOptions.class);
               return constructor.newInstance(channel, CallOptions.DEFAULT);
           } catch (Exception e) {
               throw new BeanInstantiationException(stubType, "Failed to create gRPC client via CoroutineStubFactory", e);
           }
       }
   
       @Override
       public boolean isApplicable(Class<? extends AbstractStub<?>> stubType) {
           return AbstractCoroutineStub.class.isAssignableFrom(stubType);
       }
   }
   

3. Add an auto-configuration @Bean of CoroutineStubFactory guarded by @ConditionalOnClass(AbstractCoroutineStub.class)

NOTE: That unlike the other stubs already supported, Kotlin stubs that extend AbstractCoroutineStub are generated to use constructor based instantiation, as opposed to be being instantiated via a static method.

[asarkar]

@imberda support for grpc-kotlin stubs was added via https://github.com/yidongnan/grpc-spring-boot-starter/issues/410. I’m using this library with grpc-kotlin successfully.