private MetricsCreator getMetricsCreator() {
        if (metricsCreator == null) {
            metricsCreator = moduleManager.find(TelemetryModule.NAME)
                                                          .provider()
                                                          .getService(MetricsCreator.class);
        }
        return metricsCreator;
    }
Heap memory used, jvm_memory_used_bytes{area="heap"}, Use the combination of all available ids of heap memory.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/watermark/WatermarkWatcher.java