How to intercept the headers from in call to one service and insert it to another request in gRPC-java?

I have two servers - HelloServer and WorldServer.

Both implement the same proto file:

// The greeting service definition.
service GreeterService {
    // Sends a greeting
    rpc GreetWithHelloOrWorld (GreeterRequest) returns (GreeterReply) {}
    rpc GreetWithHelloWorld (GreeterRequest) returns (GreeterReply) {}
}

message GreeterRequest {
    string id = 1;
}

// The response message containing the greetings
message GreeterReply {
    string message = 1;
    string id = 2;
}

I want to add traceIds to the requests. As far as I understand, this is achieved through adding the traceId in the Metadata object.

Here is the test I am using to check that the traceIds are passed along. The request is made to the HelloServer which in turns calls the WorldServer and then finally returns the response.

@Test
public void greetHelloWorld() {
    String traceId = UUID.randomUUID().toString();
    Metadata metadata = new Metadata();
    metadata.put(MetadataKeys.TRACE_ID_METADATA_KEY, traceId);

    Greeter.GreeterRequest greeterRequest = Greeter.GreeterRequest.newBuilder().setId(traceId).build();

    ManagedChannel channel = ManagedChannelBuilder
        .forAddress("localhost", 8080)
        .usePlaintext(true)
        .build();

    AtomicReference<Metadata> trailersCapture = new AtomicReference<>();
    AtomicReference<Metadata> headersCapture = new AtomicReference<>();
    ClientInterceptor clientInterceptor = MetadataUtils.newAttachHeadersInterceptor(metadata);
    ClientInterceptor metadataCapturingClientInterceptor = MetadataUtils.newCaptureMetadataInterceptor(headersCapture, trailersCapture);

    GreeterServiceBlockingStub blockingStub = GreeterServiceGrpc.newBlockingStub(ClientInterceptors.intercept(channel, clientInterceptor, metadataCapturingClientInterceptor));
    GreeterServiceStub asyncStub = GreeterServiceGrpc.newStub(channel);

    try {
        Greeter.GreeterReply greeterReply = blockingStub.greetWithHelloWorld(greeterRequest);
        String idInResponse = greeterReply.getId();
        String idInHeaders = headersCapture.get().get(MetadataKeys.TRACE_ID_METADATA_KEY);
        logger.info("Response from HelloService and WorldService  -- , id = {}, headers = {}", greeterReply.getMessage(), idInResponse, idInHeaders);
        assertEquals("Ids in response and header did not match", idInResponse, idInHeaders);
    } catch (StatusRuntimeException e) {
        logger.warn("Exception when calling HelloService and WorldService\n" +  e);
        fail();
    } finally {
        channel.shutdown();
    }
}

Implementation of ServerInterceptor:

@Override
public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> call, Metadata headers, ServerCallHandler<ReqT, RespT> next) {

    String traceId = headers.get(MetadataKeys.TRACE_ID_METADATA_KEY);
    logger.info("objId=" + this.toString().substring(this.toString().lastIndexOf('@')) + " Trace id -- 1=" + headers.get(MetadataKeys.TRACE_ID_METADATA_KEY));
    return next.startCall(new ForwardingServerCall.SimpleForwardingServerCall<ReqT, RespT>(call) {
        @Override
        public void sendHeaders(Metadata headers) {
            headers.put(MetadataKeys.TRACE_ID_METADATA_KEY, traceId);
            logger.info("objId=" + this.toString().substring(this.toString().lastIndexOf('@')) + " Trace id -- 2  " + headers.get(MetadataKeys.TRACE_ID_METADATA_KEY));
            super.sendHeaders(headers);
        }

        @Override
        public void sendMessage(RespT message) {
           logger.info("objId=" + this.toString().substring(this.toString().lastIndexOf('@')) + " message=" + message.toString());
            super.sendMessage(message);
        }
    }, headers);

Here is the implementation of the greetWithHelloWorld() method:

public void greetWithHelloWorld(com.comcast.manitoba.world.hello.Greeter.GreeterRequest request,
                                        io.grpc.stub.StreamObserver<com.comcast.manitoba.world.hello.Greeter.GreeterReply> responseObserver) {
        Greeter.GreeterRequest greeterRequest = Greeter.GreeterRequest.newBuilder().setId(request.getId()).build();
    
        Metadata metadata = new Metadata();
        metadata.put(MetadataKeys.TRACE_ID_METADATA_KEY, "");
    
        ManagedChannel channel = ManagedChannelBuilder.forAddress("localhost", 8081)
                .usePlaintext(true).build();
    
        AtomicReference<Metadata> trailersCapture = new AtomicReference<>();
        AtomicReference<Metadata> headersCapture = new AtomicReference<>();
        ClientInterceptor clientInterceptor = MetadataUtils.newAttachHeadersInterceptor(metadata);
        ClientInterceptor metadataCapturingClientInterceptor = MetadataUtils.newCaptureMetadataInterceptor(headersCapture, trailersCapture);
    
        GreeterServiceGrpc.GreeterServiceBlockingStub blockingStub = GreeterServiceGrpc.newBlockingStub(ClientInterceptors.intercept(channel, clientInterceptor, metadataCapturingClientInterceptor));
    
        String messageFromWorldService = "";
        String replyIdFromWorldService = "";
        try {
            Greeter.GreeterReply greeterReply = blockingStub.greetWithHelloOrWorld(greeterRequest);
            messageFromWorldService = greeterReply.getMessage();
            replyIdFromWorldService = greeterReply.getId();
            logger.info("Response from WorldService  -- {}, id = {}", messageFromWorldService, replyIdFromWorldService);
        } catch (StatusRuntimeException e) {
            logger.warn("Exception when calling HelloService\n" +  e);
        }
    
        Greeter.GreeterReply greeterReply = Greeter.GreeterReply.newBuilder().setMessage("Hello" + messageFromWorldService).setId(replyIdFromWorldService).build();
        responseObserver.onNext(greeterReply);
        responseObserver.onCompleted();
    }

The problem is in the greetWithHelloWorld() method, I don't have access to Metadata, so I cannot extract the traceId from the header and attach it to the request to the World server. However, if I put a breakpoint in that method, I can see that request object does have the traceId in it which is private to it and unaccessible.

Any ideas how can I achieve this? Also, is this the best way to do pass traceIds around? I found some references to using Context. What is the difference between Context and Metadata?