Permit autoscaling feature flag in release builds (#52088)

This commit provides a path to set register the autoscaling feature flag
in release builds, and therefore enabling autoscaling in release
builds. The primary reason that we add this is so that our release docs
tests can pass. Our release docs tests do not have infrastructure in
place to only register snippets from included portions of the docs, they
instead include all docs snippets. Since autoscaling can not be enabled
in release builds, this meant that the autoscaling snippets would fail
in the release docs tests. To address then, we need the ability to
enable autoscaling in the release docs tests which we can now do with
the system property added here. This system property will be removed
when autoscaling is ready for release.

Author: jasontedor

docs/build.gradle

@@ -45,8 +45,11 @@ testClusters.integTest {
   if (singleNode().testDistribution == DEFAULT) {
     setting 'xpack.license.self_generated.type', 'trial'
     setting 'indices.lifecycle.history_index_enabled', 'false'
+    if (BuildParams.isSnapshotBuild() == false) {
+      systemProperty 'es.autoscaling_feature_flag_registered', 'true'
+    }
+    setting 'xpack.autoscaling.enabled', 'true'
     if (BuildParams.isSnapshotBuild()) {
-      setting 'xpack.autoscaling.enabled', 'true'
       setting 'xpack.eql.enabled', 'true'
     }
   }

x-pack/plugin/autoscaling/src/main/java/org/elasticsearch/xpack/autoscaling/Autoscaling.java

@@ -33,6 +33,24 @@
  */
 public class Autoscaling extends Plugin implements ActionPlugin {
 
+    private static final boolean AUTOSCALING_FEATURE_FLAG_REGISTERED;
+
+    static {
+        final String property = System.getProperty("es.autoscaling_feature_flag_registered");
+        if (Build.CURRENT.isSnapshot() && property != null) {
+            throw new IllegalArgumentException("es.autoscaling_feature_flag_registered is only supported in non-snapshot builds");
+        }
+        if ("true".equals(property)) {
+            AUTOSCALING_FEATURE_FLAG_REGISTERED = true;
+        } else if ("false".equals(property) || property == null) {
+            AUTOSCALING_FEATURE_FLAG_REGISTERED = false;
+        } else {
+            throw new IllegalArgumentException(
+                "expected es.autoscaling_feature_flag_registered to be unset or [true|false] but was [" + property + "]"
+            );
+        }
+    }
+
     public static final Setting<Boolean> AUTOSCALING_ENABLED_SETTING = Setting.boolSetting(
         "xpack.autoscaling.enabled",
         false,
@@ -52,7 +70,7 @@ public Autoscaling(final Settings settings) {
      */
     @Override
     public List<Setting<?>> getSettings() {
-        if (isSnapshot()) {
+        if (isSnapshot() || AUTOSCALING_FEATURE_FLAG_REGISTERED) {
             return Collections.singletonList(AUTOSCALING_ENABLED_SETTING);
         } else {
             return Collections.emptyList();