Support Generational Shenandoah GC metrics (#6545)

Generational Shenandoah is an experimental feature in Java 24 and a GA feature in Java 25.
This adds support for it to JvmGcMetrics.
Run GC tests with generational Shenandoah on JDK 24 for now. When we have a JDK 25 build, we can use that and remove the experimental option.

Resolves gh-4259

Author: shakuzen

.circleci/config.yml

@@ -71,6 +71,7 @@ jobs:
     executor: jdk24-executor
     steps:
       - gradlew-build
+      - run: ./gradlew generationalShenandoahTest
 
   build:
     executor: circle-jdk-executor

micrometer-core/build.gradle

@@ -239,6 +239,17 @@ tasks.register("shenandoahTest", Test) {
     jvmArgs '-XX:+UseShenandoahGC'
 }
 
+tasks.register("generationalShenandoahTest", Test) {
+    // set heap size for the test JVM(s)
+    maxHeapSize = "1500m"
+
+    useJUnitPlatform {
+        includeTags 'gc'
+    }
+
+    jvmArgs '-XX:+UseShenandoahGC', '-XX:+UnlockExperimentalVMOptions', '-XX:ShenandoahGCMode=generational'
+}
+
 tasks.register("zgcTest", Test) {
     // set heap size for the test JVM(s)
     maxHeapSize = "1500m"

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

@@ -277,7 +277,7 @@ private boolean shouldUpdateDataSizeMetrics(String gcName) {
         }
 
         private boolean isMajorGenerationalGc(String gcName) {
-            return GcGenerationAge.fromGcName(gcName) == GcGenerationAge.OLD;
+            return isGenerationalGc && GcGenerationAge.fromGcName(gcName) == GcGenerationAge.OLD;
         }
 
         private boolean nonGenerationalGcShouldUpdateDataSize(String gcName) {
@@ -392,6 +392,7 @@ enum GcGenerationAge {
             knownCollectors.put("GPGC Old Pauses", OLD); // new naming
             // do not include 'ZGC Major Pauses'; see gh-2872
             knownCollectors.put("ZGC Major Cycles", OLD);
+            knownCollectors.put("Shenandoah Cycles", OLD);
         }
 
         static GcGenerationAge fromGcName(String gcName) {

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

@@ -45,7 +45,9 @@ static boolean isConcurrentPhase(String cause, String name) {
     }
 
     static boolean isAllocationPool(@Nullable String name) {
-        return name != null && (name.endsWith("Eden Space") || "Shenandoah".equals(name) //
+        return name != null && (name.endsWith("Eden Space") //
+                || "Shenandoah".equals(name) // non-generational Shenandoah
+                || "Shenandoah Young Gen".equals(name) // generational Shenandoah
                 || "ZHeap".equals(name) // ZGC non-generational
                 || "ZGC Young Generation".equals(name) // generational ZGC
                 || name.endsWith("New Gen") // Zing GPGC
@@ -55,7 +57,9 @@ static boolean isAllocationPool(@Nullable String name) {
     }
 
     static boolean isLongLivedPool(@Nullable String name) {
-        return name != null && (name.endsWith("Old Gen") || name.endsWith("Tenured Gen") || "Shenandoah".equals(name)
+        return name != null && (name.endsWith("Old Gen") || name.endsWith("Tenured Gen") //
+                || "Shenandoah".equals(name) // non-generational Shenandoah
+                || "Shenandoah Old Generation".equals(name) // generational Shenandoah
                 || "ZHeap".equals(name) // ZGC non-generational
                 || "ZGC Old Generation".equals(name) // generational ZGC
                 || name.endsWith("balanced-old") //