Align thread pool info to thread pool configuration (#29123)

Today we report thread pool info using a common object. This means that
we use a shared set of terminology that is not consistent with the
terminology used to the configure thread pools. This holds in particular
for the minimum and maximum number of threads in the thread pool where
we use the following terminology:
 thread pool info | fixed | scaling
 min                core    size
 max                max     size

This commit changes the display of thread pool info to be dependent on
the type of the thread pool so that we can align the terminology in the
output of thread pool info with the terminology used to configure a
thread pool.

server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java

@@ -58,6 +58,7 @@
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
+import java.util.stream.Collectors;
 
 import static java.util.Collections.unmodifiableMap;
 
@@ -138,7 +139,9 @@ public static ThreadPoolType fromType(String type) {
         THREAD_POOL_TYPES = Collections.unmodifiableMap(map);
     }
 
-    private Map<String, ExecutorHolder> executors = new HashMap<>();
+    private final Map<String, ExecutorHolder> executors;
+
+    private final ThreadPoolInfo threadPoolInfo;
 
     private final CachedTimeThread cachedTimeThread;
 
@@ -207,6 +210,15 @@ public ThreadPool(final Settings settings, final ExecutorBuilder<?>... customBui
 
         executors.put(Names.SAME, new ExecutorHolder(DIRECT_EXECUTOR, new Info(Names.SAME, ThreadPoolType.DIRECT)));
         this.executors = unmodifiableMap(executors);
+
+        final List<Info> infos =
+                executors
+                        .values()
+                        .stream()
+                        .filter(holder -> holder.info.getName().equals("same") == false)
+                        .map(holder -> holder.info)
+                        .collect(Collectors.toList());
+        this.threadPoolInfo = new ThreadPoolInfo(infos);
         this.scheduler = Scheduler.initScheduler(settings);
         TimeValue estimatedTimeInterval = ESTIMATED_TIME_INTERVAL_SETTING.get(settings);
         this.cachedTimeThread = new CachedTimeThread(EsExecutors.threadName(settings, "[timer]"), estimatedTimeInterval.millis());
@@ -239,16 +251,7 @@ public Counter estimatedTimeInMillisCounter() {
     }
 
     public ThreadPoolInfo info() {
-        List<Info> infos = new ArrayList<>();
-        for (ExecutorHolder holder : executors.values()) {
-            String name = holder.info.getName();
-            // no need to have info on "same" thread pool
-            if ("same".equals(name)) {
-                continue;
-            }
-            infos.add(holder.info);
-        }
-        return new ThreadPoolInfo(infos);
+        return threadPoolInfo;
     }
 
     public Info info(String name) {
@@ -655,32 +658,29 @@ public SizeValue getQueueSize() {
         @Override
         public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
             builder.startObject(name);
-            builder.field(Fields.TYPE, type.getType());
-            if (min != -1) {
-                builder.field(Fields.MIN, min);
-            }
-            if (max != -1) {
-                builder.field(Fields.MAX, max);
+            builder.field("type", type.getType());
+
+            if (type == ThreadPoolType.SCALING) {
+                assert min != -1;
+                builder.field("core", min);
+                assert max != -1;
+                builder.field("max", max);
+            } else {
+                assert max != -1;
+                builder.field("size", max);
             }
             if (keepAlive != null) {
-                builder.field(Fields.KEEP_ALIVE, keepAlive.toString());
+                builder.field("keep_alive", keepAlive.toString());
             }
             if (queueSize == null) {
-                builder.field(Fields.QUEUE_SIZE, -1);
+                builder.field("queue_size", -1);
             } else {
-                builder.field(Fields.QUEUE_SIZE, queueSize.singles());
+                builder.field("queue_size", queueSize.singles());
             }
             builder.endObject();
             return builder;
         }
 
-        static final class Fields {
-            static final String TYPE = "type";
-            static final String MIN = "min";
-            static final String MAX = "max";
-            static final String KEEP_ALIVE = "keep_alive";
-            static final String QUEUE_SIZE = "queue_size";
-        }
     }
 
     /**