Ignore ZGC warmup + zero-sized live data after GC

In some cases, the live data after GC size in the GC notification is
reported as 0 when ZGC is warming up.
We should ignore these notifications.

Closes gh-4497

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/jvm/JvmGcMetrics.java

@@ -219,11 +219,11 @@ public void handleNotification(Notification notification, Object ref) {
 
             // Some GC implementations such as G1 can reduce the old gen size as part of a
             // minor GC. To track the live data size we record the value if we see a
-            // reduction in the long-lived
-            // heap size or after a major/non-generational GC. In some cases,
-            // longLivedAfter is 0, we ignore those notifications, see: gh-4497.
-            if (longLivedAfter > 0 && (longLivedAfter < longLivedBefore
-                    || shouldUpdateDataSizeMetrics(notificationInfo.getGcName()))) {
+            // reduction in the long-lived heap size or after a major/non-generational GC.
+            // ZGC Warmup notifications should be ignored since the long-lived heap size
+            // is zero there.
+            if ((longLivedAfter < longLivedBefore || shouldUpdateDataSizeMetrics(notificationInfo.getGcName()))
+                    && !isZgcWarmingUp(gcCause, longLivedAfter)) {
                 liveDataSize.set(longLivedAfter);
                 maxDataSize.set(longLivedPoolNames.stream().mapToLong(pool -> after.get(pool).getMax()).sum());
             }
@@ -242,6 +242,19 @@ private void countPoolSizeDelta(Map<String, MemoryUsage> before, Map<String, Mem
             }
         }
 
+        /**
+         * Sometimes ZGC emits "Warmup" notification, where longLivedAfter is 0, we should
+         * ignore those notifications, see: gh-4497. This method should detect these
+         * cases.
+         * @param gcCause The action performed by the garbage collector
+         * @param longLivedAfter Size of the long-lived pools after collection
+         * @return Whether the ZGC warmup notification is emitted with zero long-lived
+         * pools size
+         */
+        private boolean isZgcWarmingUp(String gcCause, long longLivedAfter) {
+            return longLivedAfter == 0 && "Warmup".equals(gcCause);
+        }
+
         private boolean shouldUpdateDataSizeMetrics(String gcName) {
             return nonGenerationalGcShouldUpdateDataSize(gcName) || isMajorGenerationalGc(gcName);
         }