Avoid redundantly copying a map in Settings builder (#135187)

When putting an existing `Settings` object into a `Settings.Builder`, we
currently check if there are any legacy list setting keys (by checking
if there's one key ending in `.0`). The value of doing that there in
addition to doing it in the final `build()` method seems to be that
legacy settings can be overriden by subsequent calls to the same
builder, rather than the legacy settings overriding others at the end.

Instead of copying the existing settings' map to a temporary map, we can
put the keys directly into the builder's map and perform the legacy
settings check there.

Author: nielsbauman

server/src/main/java/org/elasticsearch/common/settings/Settings.java

@@ -1191,9 +1191,8 @@ public Builder put(Settings settings) {
          * @param copySecureSettings if <code>true</code> all settings including secure settings are copied.
          */
         public Builder put(Settings settings, boolean copySecureSettings) {
-            Map<String, Object> settingsMap = new HashMap<>(settings.settings);
-            processLegacyLists(settingsMap);
-            map.putAll(settingsMap);
+            map.putAll(settings.settings);
+            processLegacyLists(map);
             if (copySecureSettings && settings.getSecureSettings() != null) {
                 setSecureSettings(settings.getSecureSettings());
             }