Represent lists as actual lists inside Settings (#26878)

Today we represent each value of a list setting with it's own dedicated key
that ends with the index of the value in the list. Aside of the obvious
weirdness this has several issues especially if lists are massive since it
causes massive runtime penalties when validating settings. Like a list of 100k
words will literally cause a create index call to timeout and in-turn massive
slowdown on all subsequent validations runs.

With this change we use a simple string list to represent the list. This change
also forbids to add a settings that ends with a .0 which was internally used to
detect a list setting.  Once this has been rolled out for an entire major
version all the internal .0 handling can be removed since all settings will be
converted.

Relates to #26723

Author: s1monw

core/src/main/java/org/elasticsearch/common/settings/AbstractScopedSettings.java

@@ -86,7 +86,7 @@ protected AbstractScopedSettings(Settings settings, Set<Setting<?>> settingsSet,
 
     protected void validateSettingKey(Setting setting) {
         if (isValidKey(setting.getKey()) == false && (setting.isGroupSetting() && isValidGroupKey(setting.getKey())
-            || isValidAffixKey(setting.getKey())) == false) {
+            || isValidAffixKey(setting.getKey())) == false || setting.getKey().endsWith(".0")) {
             throw new IllegalArgumentException("illegal settings key: [" + setting.getKey() + "]");
         }
     }
@@ -534,7 +534,7 @@ private static boolean applyDeletes(Set<String> deletes, Settings.Builder builde
         boolean changed = false;
         for (String entry : deletes) {
             Set<String> keysToRemove = new HashSet<>();
-            Set<String> keySet = builder.internalMap().keySet();
+            Set<String> keySet = builder.keys();
             for (String key : keySet) {
                 if (Regex.simpleMatch(entry, key) && canRemove.test(key)) {
                     // we have to re-check with canRemove here since we might have a wildcard expression foo.* that matches

core/src/main/java/org/elasticsearch/common/settings/ClusterSettings.java

@@ -127,7 +127,7 @@ public Settings getValue(Settings current, Settings previous) {
             Settings.Builder builder = Settings.builder();
             builder.put(current.filter(loggerPredicate));
             for (String key : previous.keySet()) {
-                if (loggerPredicate.test(key) && builder.internalMap().containsKey(key) == false) {
+                if (loggerPredicate.test(key) && builder.keys().contains(key) == false) {
                     if (ESLoggerFactory.LOG_LEVEL_SETTING.getConcreteSetting(key).exists(settings) == false) {
                         builder.putNull(key);
                     } else {

core/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -820,12 +820,6 @@ boolean hasComplexMatcher() {
             return true;
         }
 
-        @Override
-        public boolean exists(Settings settings) {
-            boolean exists = super.exists(settings);
-            return exists || settings.get(getKey() + ".0") != null;
-        }
-
         @Override
         public void diff(Settings.Builder builder, Settings source, Settings defaultSettings) {
             if (exists(source) == false) {