private long heapMemoryUsagePercent() {
        if (heapMemoryMax > 0) {
            return heapMemoryUsed * 100 / heapMemoryMax;
        } else {
            return -1;
        }
    }
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