Assert write thread pool stats are present

Author: nicktindall

server/src/main/java/org/elasticsearch/cluster/routing/allocation/WriteLoadConstraintMonitor.java

@@ -81,6 +81,7 @@ public void onNewInfo(ClusterInfo clusterInfo) {
         clusterInfo.getNodeUsageStatsForThreadPools().forEach((nodeId, usageStats) -> {
             final NodeUsageStatsForThreadPools.ThreadPoolUsageStats writeThreadPoolStats = usageStats.threadPoolUsageStatsMap()
                 .get(ThreadPool.Names.WRITE);
+            assert writeThreadPoolStats != null : "Write thread pool is not publishing usage stats for node [" + nodeId + "]";
             if (writeThreadPoolStats.maxThreadPoolQueueLatencyMillis() > writeLoadConstraintSettings.getQueueLatencyThreshold().millis()) {
                 nodeIdsExceedingLatencyThreshold.add(nodeId);
             }