@Override
    public void export(ExportEvent event) {
        Metrics metrics = event.getMetrics();
        if (metrics instanceof WithMetadata) {
            MetricsMetaInfo meta = ((WithMetadata) metrics).getMeta();
            if (subscriptionList.size() == 0 && ExportEvent.EventType.INCREMENT.equals(event.getType())) {
                exportBuffer.produce(new ExportData(meta, metrics, event.getType()));
            } else {
                subscriptionList.forEach(subscriptionMetric -> {
                    if (subscriptionMetric.getMetricName().equals(meta.getMetricsName()) &&
                        eventTypeMatch(event.getType(), subscriptionMetric.getEventType())) {
                        exportBuffer.produce(new ExportData(meta, metrics, event.getType()));
                    }
                });
            }

            fetchSubscriptionList();
        }
    }
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