Lazy load stats in NodeMetrics (#113014)

No point in eager loading the metrics here, it just slows down node
startup considerably, wasting lots of time in integTest runs in
particular (as in e.g. ~200s of total CPU time in
:server:internalClusterTest!!).

Author: original-brownbear

server/src/main/java/org/elasticsearch/monitor/metrics/NodeMetrics.java

@@ -13,7 +13,10 @@
 import org.apache.logging.log4j.Logger;
 import org.elasticsearch.action.admin.cluster.node.stats.NodeStats;
 import org.elasticsearch.action.admin.indices.stats.CommonStatsFlags;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.node.VersionInformation;
 import org.elasticsearch.common.component.AbstractLifecycleComponent;
+import org.elasticsearch.common.transport.TransportAddress;
 import org.elasticsearch.common.util.SingleObjectCache;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.stats.IndexingPressureStats;
@@ -24,9 +27,12 @@
 import org.elasticsearch.telemetry.metric.MeterRegistry;
 
 import java.io.IOException;
+import java.net.InetAddress;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 import java.util.Optional;
+import java.util.Set;
 
 /**
  * NodeMetrics monitors various statistics of an Elasticsearch node and exposes them as metrics through
@@ -744,10 +750,38 @@ protected void doClose() throws IOException {
      * refresh() is called, cache is updated and the new instance returned.
      */
     private class NodeStatsCache extends SingleObjectCache<NodeStats> {
+        private static final NodeStats MISSING_NODE_STATS = new NodeStats(
+            new DiscoveryNode(
+                "_na",
+                "_na",
+                new TransportAddress(InetAddress.getLoopbackAddress(), 0),
+                Map.of(),
+                Set.of(),
+                VersionInformation.CURRENT
+            ),
+            0,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null,
+            null
+        );
         private boolean refresh;
 
         NodeStatsCache(TimeValue interval) {
-            super(interval, getNodeStats());
+            super(interval, MISSING_NODE_STATS);
             this.refresh = true;
         }
 
@@ -756,6 +790,11 @@ protected NodeStats refresh() {
             return refresh ? getNodeStats() : getNoRefresh();
         }
 
+        @Override
+        protected boolean needsRefresh() {
+            return getNoRefresh() == MISSING_NODE_STATS || super.needsRefresh();
+        }
+
         public void stopRefreshing() {
             this.refresh = false;
         }