@Override
    public void start() {
        GRPCClient client = new GRPCClient(setting.getGRPCTargetHost(), setting.getGRPCTargetPort());
        client.connect();
        ManagedChannel channel = client.getChannel();
        exportServiceFutureStub = MetricExportServiceGrpc.newStub(channel);
        blockingStub = MetricExportServiceGrpc.newBlockingStub(channel);
        exportBuffer = new DataCarrier<ExportData>(setting.getBufferChannelNum(), setting.getBufferChannelSize());
        exportBuffer.consume(this, 1, 200);
        subscriptionList = new ArrayList<>();
        fetchListLock = new ReentrantLock();
    }
The period of subscription list fetching is hardcoded as 30s.
oap-server/exporter/src/main/java/org/apache/skywalking/oap/server/exporter/provider/grpc/GRPCMetricsExporter.java