Add option to the protoc code generator to avoid generating deprecate…

…d code (#3089)

### Motivation

The generated ServiceTalk gRPC stubs make use of deprecated calls. This causes a problem when `-Werror` is used to build the code since it will automatically fail the build. We should allow users who have already migrated their code to prevent the protoc compiler from generating and using deprecated references.

### Modifications

Add an option, `skipDeprecated`, as part of the protoc compiler configuration, to tell the generator to leave out deprecated references.

Remove
- initSerializationProvider, reason: ContentCodec is deprecated
- isSupportedMessageCodingsEmpty, reason ContentCodec is deprecated
- Deprecated ServiceFactory constructors
- ServiceFactory::Builder references to ContentCodec
- Generated client metadata and associated methods, reason: deprecated

### Result

Users can generate gRPC stubs without deprecated code.

### Testing

Manually tested:
- Add new option to ServiceTalk/examples/grpc/protoc-options and use it to generate test_service.proto which covers all combinations of streaming (or not) services.

servicetalk-examples/grpc/protoc-options/build.gradle

@@ -74,6 +74,8 @@ protobuf {
           outputSubDir = "java"
           // Option to append a suffix to type names to avoid naming collisions with other code generation.
           option 'typeNameSuffix=St'
+          // Option to tell the compiler to exclude all Deprecated fields, types, and methods from the output
+          option 'skipDeprecated=true'
         }
       }
     }

servicetalk-grpc-api/src/main/java/io/servicetalk/grpc/api/GrpcRoutes.java

@@ -138,7 +138,9 @@ final Single<GrpcServerContext> bind(final ServerBinder binder, final GrpcExecut
      * In case you have a use-case, let us know.
      */
     @Deprecated
-    protected abstract void registerRoutes(Service service);    // FIXME: 0.43 - remove deprecated method
+    protected void registerRoutes(Service service) {    // FIXME: 0.43 - remove deprecated method
+        throw new UnsupportedOperationException("This method is not used starting from version 0.42.0");
+    }
 
     /**
      * Create a new {@link Service} from the passed {@link AllGrpcRoutes}.
@@ -150,7 +152,9 @@ final Single<GrpcServerContext> bind(final ServerBinder binder, final GrpcExecut
      * In case you have a use-case, let us know.
      */
     @Deprecated
-    protected abstract Service newServiceFromRoutes(AllGrpcRoutes routes);  // FIXME: 0.43 - remove deprecated method
+    protected Service newServiceFromRoutes(AllGrpcRoutes routes) {  // FIXME: 0.43 - remove deprecated method
+        throw new UnsupportedOperationException("This method is not used starting from version 0.42.0");
+    }
 
     static GrpcRoutes<?> merge(GrpcRoutes<?>... allRoutes) {
         final GrpcRouter.Builder[] builders = new GrpcRouter.Builder[allRoutes.length];

servicetalk-grpc-protoc/README.adoc

@@ -22,7 +22,7 @@ link:{source-root}/servicetalk-examples/grpc/helloworld[gRPC HelloWorld Example]
 This plugin supports the following
 link:https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.compiler.command_line_interface[options]:
 
-==== `javaDocs=<true|false>`
+==== `javaDocs=<true|false>`  (default: true)
 Control if `javadoc` is generated by this plugin. Here is an example of how you can specify this option with gradle:
 
 [source,gradle]
@@ -47,7 +47,7 @@ And with Maven:
 </protocPlugin>
 ----
 
-==== `typeNameSuffix=<value>`
+==== `typeNameSuffix=<value>` (default: not enabled)
 Appends the <value> onto service type names generated by this plugin. This helps to avoid service type name
 collisions if other protoc plugins are generating code in the same gradle project, or in the same package name.
 
@@ -78,3 +78,36 @@ And with Maven:
   </args>
 </protocPlugin>
 ----
+
+==== `skipDeprecated=<true|false>` (default: false)
+Instructs the generator to remove all deprecated methods and fields from the generated code, as well as remove calls
+to deprecated code either to internal ServiceTalk libraries, or outside. This will almost certainly cause breaking changes
+to code that uses these deprecated methods but will be forward compatible.
+
+If you are using the
+link:https://github.com/google/protobuf-gradle-plugin#configure-what-to-generate[protobuf-gradle-plugin] this is how you
+can specify an option:
+
+[source,gradle]
+----
+task.plugins {
+  servicetalk_grpc {
+    option 'skipDeprecated=true'
+  }
+}
+----
+
+And with Maven:
+
+And with Maven:
+
+[source, xml]
+----
+<protocPlugin>
+  <id>servicetalk-grpc-protoc</id>
+  <!-- more params -->
+  <args>
+    <arg>skipDeprecated=true</arg>
+  </args>
+</protocPlugin>
+----