Speedup OsStats initialization (#118141) (#118209)

Similar to other OS/FS type stats we can optimize here. Found this
as a slowdown when profiling tests in a loop during test fixing.
This helps node startup and maybe more importantly test performance.
No need to initialize the stats eagerly when we can just get them
as we load them the first time.

Author: original-brownbear

server/src/main/java/org/elasticsearch/monitor/os/OsService.java

@@ -25,7 +25,6 @@ public class OsService implements ReportingService<OsInfo> {
 
     private static final Logger logger = LogManager.getLogger(OsService.class);
 
-    private final OsProbe probe;
     private final OsInfo info;
     private final SingleObjectCache<OsStats> osStatsCache;
 
@@ -37,10 +36,9 @@ public class OsService implements ReportingService<OsInfo> {
     );
 
     public OsService(Settings settings) throws IOException {
-        this.probe = OsProbe.getInstance();
         TimeValue refreshInterval = REFRESH_INTERVAL_SETTING.get(settings);
-        this.info = probe.osInfo(refreshInterval.millis(), EsExecutors.nodeProcessors(settings));
-        this.osStatsCache = new OsStatsCache(refreshInterval, probe.osStats());
+        this.info = OsProbe.getInstance().osInfo(refreshInterval.millis(), EsExecutors.nodeProcessors(settings));
+        this.osStatsCache = new OsStatsCache(refreshInterval);
         logger.debug("using refresh_interval [{}]", refreshInterval);
     }
 
@@ -53,14 +51,28 @@ public OsStats stats() {
         return osStatsCache.getOrRefresh();
     }
 
-    private class OsStatsCache extends SingleObjectCache<OsStats> {
-        OsStatsCache(TimeValue interval, OsStats initValue) {
-            super(interval, initValue);
+    private static class OsStatsCache extends SingleObjectCache<OsStats> {
+
+        private static final OsStats MISSING = new OsStats(
+            0L,
+            new OsStats.Cpu((short) 0, new double[0]),
+            new OsStats.Mem(0, 0, 0),
+            new OsStats.Swap(0, 0),
+            null
+        );
+
+        OsStatsCache(TimeValue interval) {
+            super(interval, MISSING);
         }
 
         @Override
         protected OsStats refresh() {
-            return probe.osStats();
+            return OsProbe.getInstance().osStats();
+        }
+
+        @Override
+        protected boolean needsRefresh() {
+            return getNoRefresh() == MISSING || super.needsRefresh();
         }
     }
 }