[ISSUE #3949] allow to add user-defined gRPC Server interceptors.

proxy/src/main/java/org/apache/rocketmq/proxy/grpc/GrpcServerBuilder.java

@@ -17,6 +17,7 @@
 package org.apache.rocketmq.proxy.grpc;
 
 import io.grpc.BindableService;
+import io.grpc.ServerInterceptor;
 import io.grpc.ServerServiceDefinition;
 import io.grpc.netty.shaded.io.grpc.netty.GrpcSslContexts;
 import io.grpc.netty.shaded.io.grpc.netty.NettyServerBuilder;
@@ -100,6 +101,11 @@ public class GrpcServerBuilder {
         return this;
     }
 
+    public GrpcServerBuilder appendInterceptor(ServerInterceptor interceptor) {
+        this.serverBuilder.intercept(interceptor);
+        return this;
+    }
+
     public GrpcServer build() {
         return new GrpcServer(this.serverBuilder.build());
     }

proxy/src/main/java/org/apache/rocketmq/proxy/grpc/interceptor/InterceptorConstants.java

@@ -21,9 +21,6 @@ import io.grpc.Context;
 import io.grpc.Metadata;
 
 public class InterceptorConstants {
-    private InterceptorConstants() {
-    }
-
     public static final Context.Key<Metadata> METADATA = Context.key("rpc-metadata");
 
     /**