如何为grpc-java客户端指定User-Agent头部？

import io.grpc.*;

public class UserAgentInterceptor implements ClientInterceptor {
    private final Metadata.Key<String> userAgentKey = Metadata.Key.of("User-Agent", Metadata.ASCII_STRING_MARSHALLER);
    private final String userAgentValue;

    public UserAgentInterceptor(String userAgentValue) {
        this.userAgentValue = userAgentValue;
    }

    @Override
    public <ReqT, RespT> ClientCall<ReqT, RespT> interceptCall(
            MethodDescriptor<ReqT, RespT> method,
            CallOptions callOptions,
            Channel next) {
        ClientCall<ReqT, RespT> call = next.newCall(method, callOptions);
        call = new ForwardingClientCall.SimpleForwardingClientCall<ReqT, RespT>(call) {
            @Override
            public void start(Listener<RespT> responseListener, Metadata headers) {
                headers.put(userAgentKey, userAgentValue);
                super.start(responseListener, headers);
            }
        };
        return call;
    }
}

import io.grpc.*;

public class GrpcClient {
    private final ManagedChannel channel;
    private final GreeterGrpc.GreeterBlockingStub blockingStub;

    public GrpcClient(String host, int port, String userAgent) {
        channel = ManagedChannelBuilder.forAddress(host, port)
                .intercept(new UserAgentInterceptor(userAgent))
                .usePlaintext()
                .build();
        blockingStub = GreeterGrpc.newBlockingStub(channel);
    }

    // ...
}