title: Protobuf plugin for gradle does not generate service class
body: <p>I try to build a gRPC  and protobuf application in java with Gradle</p>
  
  <p>I followed the instruction from: <a href="https://github.com/grpc/grpc-java/blob/master/README.md" rel="noreferrer">https://github.com/grpc/grpc-java/blob/master/README.md</a></p>
  
  <p>The issue is that one file is not generated: *ServiceGrpc.java
  But the corresponding *ServiceGrpc.class file is in the build directory generated by the gradle build.</p>
  
  <p>I tried with running the compiler manually with the command protoc but I have the exact same issue (I'm on Ubuntu 18.04)</p>
  
  <p>Here is my proto file</p>
  
  <pre><code>syntax = "proto3";
  
  option java_multiple_files=true; 
  option java_generic_services= true;
  //...//
  message Track { 
      int64 id = 1; //... }
  service TrackService { 
      rpc Create(Track) returns (Response); }
  //...
  </code></pre>
  
  <p>The file Track.java, TrackOrBuilder.java, TrackOuterClass.java are all there. As well as their corresponding .class files in the build directory.</p>
  
  <p>With the flag "option java_generic_services= true", TrackService.java is generated, and again .class file.</p>
  
  <p>But not matter what, the file TrackServiceGrpc.java is not created, contrary its correspond .class file, which is quite confusing.</p>
  
  <p>here is my build.gradle :</p>
  
  <pre><code>buildscript {
      repositories {
          mavenCentral()
      }
      dependencies {
          classpath 'com.google.protobuf:protobuf-gradle-plugin:0.8.8'
      }
  }
  plugins {
      id 'java'
  }
  repositories {
      mavenCentral()
  }
  apply plugin: 'com.google.protobuf'
  
  protobuf {
      protoc {
          artifact = "com.google.protobuf:protoc:3.9.0"
          generateProtoTasks.generatedFilesBaseDir = 'src'
      }
      plugins {
          grpc {
              artifact = 'io.grpc:protoc-gen-grpc-java:1.23.0'
          }
      }
  
      generateProtoTasks {
          all()*.plugins {
              grpc {}
          }
      }
  }
  
  dependencies {
      testCompile group: 'junit', name: 'junit', version: '4.12'
      //https://github.com/grpc/grpc-java/blob/master/README.md
      implementation 'io.grpc:grpc-netty-shaded:1.23.0'
      implementation 'io.grpc:grpc-protobuf:1.23.0'
      implementation 'io.grpc:grpc-stub:1.23.0'
      compile group: 'javax.annotation', name: 'javax.annotation-api', version: '1.3.2'
  }
  </code></pre>
  
  <p>What am I doing wrong ?</p>