Mark more server-side pieces experimental

You can't make an interceptor without calling ServerCallHandler, so it
is now marked experimental.

core/src/main/java/io/grpc/ServerInterceptor.java

@@ -46,6 +46,7 @@ import javax.annotation.concurrent.ThreadSafe;
  * </ul>
  */
 @ThreadSafe
+@ExperimentalApi
 public interface ServerInterceptor {
   /**
    * Intercept {@link ServerCall} dispatch by the {@code next} {@link ServerCallHandler}. General

core/src/main/java/io/grpc/ServerMethodDefinition.java

@@ -36,6 +36,7 @@ package io.grpc;
  *
  * @see ServerServiceDefinition
  */
+@ExperimentalApi
 public final class ServerMethodDefinition<ReqT, RespT> {
   private final MethodDescriptor<ReqT, RespT> method;
   private final ServerCallHandler<ReqT, RespT> handler;

core/src/main/java/io/grpc/ServerServiceDefinition.java

@@ -46,6 +46,7 @@ import java.util.Map;
 // consider removing ServerServiceDefinition to and let the registry to have a big map of
 // handlers.
 public final class ServerServiceDefinition {
+  @ExperimentalApi
   public static Builder builder(String serviceName) {
     return new Builder(serviceName);
   }
@@ -64,6 +65,7 @@ public final class ServerServiceDefinition {
     return name;
   }
 
+  @ExperimentalApi
   public Collection<ServerMethodDefinition<?, ?>> getMethods() {
     return methods.values();
   }
@@ -93,6 +95,7 @@ public final class ServerServiceDefinition {
      * @param method the {@link MethodDescriptor} of this method.
      * @param handler handler for incoming calls
      */
+    @ExperimentalApi
     public <ReqT, RespT> Builder addMethod(
         MethodDescriptor<ReqT, RespT> method, ServerCallHandler<ReqT, RespT> handler) {
       return addMethod(ServerMethodDefinition.create(
@@ -101,6 +104,7 @@ public final class ServerServiceDefinition {
     }
 
     /** Add a method to be supported by the service. */
+    @ExperimentalApi
     public <ReqT, RespT> Builder addMethod(ServerMethodDefinition<ReqT, RespT> def) {
       MethodDescriptor<ReqT, RespT> method = def.getMethodDescriptor();
       checkArgument(