Enable deprecation checks for removed settings (#53317)

Today we do not have any infrastructure for adding a deprecation check
for settings that are removed. This commit enables this by adding such
infrastructure. Note that this infrastructure is unused in this commit,
which is deliberate. However, the primary target for this commit is 7.x
where this infrastructue will be used, in a follow-up.

x-pack/plugin/deprecation/src/main/java/org/elasticsearch/xpack/deprecation/NodeDeprecationChecks.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.deprecation;
+
+import org.elasticsearch.common.settings.Setting;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
+
+import java.util.Locale;
+
+public class NodeDeprecationChecks {
+
+    static DeprecationIssue checkRemovedSetting(final Settings settings, final Setting<?> removedSetting, final String url) {
+        if (removedSetting.exists(settings) == false) {
+            return null;
+        }
+        final String removedSettingKey = removedSetting.getKey();
+        final String value = removedSetting.get(settings).toString();
+        final String message =
+            String.format(Locale.ROOT, "setting [%s] is deprecated and will be removed in the next major version", removedSettingKey);
+        final String details =
+            String.format(Locale.ROOT, "the setting [%s] is currently set to [%s], remove this setting", removedSettingKey, value);
+        return new DeprecationIssue(DeprecationIssue.Level.CRITICAL, message, url, details);
+    }
+
+}

x-pack/plugin/deprecation/src/test/java/org/elasticsearch/xpack/deprecation/NodeDeprecationChecksTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.deprecation;
+
+import org.elasticsearch.common.settings.Setting;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xpack.core.deprecation.DeprecationIssue;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.not;
+import static org.hamcrest.Matchers.nullValue;
+
+public class NodeDeprecationChecksTests extends ESTestCase {
+
+    public void testRemovedSettingNotSet() {
+        final Settings settings = Settings.EMPTY;
+        final Setting<?> removedSetting = Setting.simpleString("node.removed_setting");
+        final DeprecationIssue issue =
+            NodeDeprecationChecks.checkRemovedSetting(settings, removedSetting, "http://removed-setting.example.com");
+        assertThat(issue, nullValue());
+    }
+
+    public void testRemovedSetting() {
+        final Settings settings = Settings.builder().put("node.removed_setting", "value").build();
+        final Setting<?> removedSetting = Setting.simpleString("node.removed_setting");
+        final DeprecationIssue issue =
+            NodeDeprecationChecks.checkRemovedSetting(settings, removedSetting, "https://removed-setting.example.com");
+        assertThat(issue, not(nullValue()));
+        assertThat(issue.getLevel(), equalTo(DeprecationIssue.Level.CRITICAL));
+        assertThat(
+            issue.getMessage(),
+            equalTo("setting [node.removed_setting] is deprecated and will be removed in the next major version"));
+        assertThat(
+            issue.getDetails(),
+            equalTo("the setting [node.removed_setting] is currently set to [value], remove this setting"));
+        assertThat(issue.getUrl(), equalTo("https://removed-setting.example.com"));
+    }
+
+}