[Monitoring] Clean .watcher-history-* indices by default in 6.3+ (elastic/x-pack-elasticsearch#2720)

This flips the setting default to true for 6.3+.

Original commit: elastic/x-pack-elasticsearch@47a09d8449

docs/en/settings/notification-settings.asciidoc

@@ -35,13 +35,15 @@ required. For more information, see
 {xpack-ref}/encrypting-data.html[Encrypting sensitive data in {watcher}].
 
 `xpack.watcher.history.cleaner_service.enabled`::
-Set to `false` (default) to disable the cleaner service. If this setting is
-`true`, the `xpack.monitoring.enabled` setting must also be set to `true`. The
-cleaner service removes previous versions of {watcher} indices (for example,
-`.watcher-history*`) when it determines that they are old. The duration of
-{watcher} indices is determined by the `xpack.monitoring.history.duration`
-setting, which defaults to 7 days. For more information about that setting,
-see <<monitoring-settings>>.
+added[6.3.0,Default changed to `true`.]
++
+Set to `true` (default) to enable the cleaner service. If this setting is
+`true`, the `xpack.monitoring.enabled` setting must also be set to `true` with
+a local exporter enabled. The cleaner service removes previous versions of
+{watcher} indices (for example, `.watcher-history*`) when it determines that
+they are old. The duration of {watcher} indices is determined by the
+`xpack.monitoring.history.duration` setting, which defaults to 7 days. For
+more information about that setting, see <<monitoring-settings>>.
 
 `xpack.http.proxy.host`::
 Specifies the address of the proxy server to use to connect to HTTP services.

plugin/monitoring/src/main/java/org/elasticsearch/xpack/monitoring/Monitoring.java

@@ -77,7 +77,7 @@ public class Monitoring extends Plugin implements ActionPlugin {
      * The ability to automatically cleanup ".watcher_history*" indices while also cleaning up Monitoring indices.
      */
     public static final Setting<Boolean> CLEAN_WATCHER_HISTORY = boolSetting("xpack.watcher.history.cleaner_service.enabled",
-                                                                             false,
+                                                                             true,
                                                                              Setting.Property.Dynamic, Setting.Property.NodeScope);
 
     protected final Settings settings;