Add missing aggregation system variables.

Move inner class SystemVariable to upper level and add missing values (NOW, CLUSTER_TIME, DECEND, PRUNE, KEEP & SEARCH_META)

Original pull request: #4176.
Closes #4145

Author: christophstrobl

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java

@@ -24,7 +24,6 @@
 import java.util.Map;
 
 import org.bson.Document;
-import org.springframework.data.mongodb.core.aggregation.Aggregation.SystemVariable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -35,7 +35,6 @@
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.SerializationUtils;
-import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
 /**
@@ -766,48 +765,4 @@ public Document toDocument(String inputCollectionName, AggregationOperationConte
 	public String toString() {
 		return SerializationUtils.serializeToJsonSafely(toDocument("__collection__", DEFAULT_CONTEXT));
 	}
-
-	/**
-	 * Describes the system variables available in MongoDB aggregation framework pipeline expressions.
-	 *
-	 * @author Thomas Darimont
-	 * @author Christoph Strobl
-	 * @see <a href="https://docs.mongodb.com/manual/reference/aggregation-variables">Aggregation Variables</a>.
-	 */
-	enum SystemVariable {
-
-		ROOT, CURRENT, REMOVE;
-
-		private static final String PREFIX = "$$";
-
-		/**
-		 * Return {@literal true} if the given {@code fieldRef} denotes a well-known system variable, {@literal false}
-		 * otherwise.
-		 *
-		 * @param fieldRef may be {@literal null}.
-		 * @return {@literal true} if the given field refers to a {@link SystemVariable}.
-		 */
-		public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
-
-			if (fieldRef == null || !fieldRef.startsWith(PREFIX) || fieldRef.length() <= 2) {
-				return false;
-			}
-
-			int indexOfFirstDot = fieldRef.indexOf('.');
-			String candidate = fieldRef.substring(2, indexOfFirstDot == -1 ? fieldRef.length() : indexOfFirstDot);
-
-			for (SystemVariable value : values()) {
-				if (value.name().equals(candidate)) {
-					return true;
-				}
-			}
-
-			return false;
-		}
-
-		@Override
-		public String toString() {
-			return PREFIX.concat(name());
-		}
-	}
 }

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Fields.java

@@ -245,7 +245,7 @@ public AggregationField(String name, @Nullable String target) {
 
 		private static String cleanUp(String source) {
 
-			if (Aggregation.SystemVariable.isReferingToSystemVariable(source)) {
+			if (SystemVariable.isReferingToSystemVariable(source)) {
 				return source;
 			}
 

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java

@@ -510,7 +510,7 @@ public Object getValue(AggregationOperationContext context) {
 				return value;
 			}
 
-			if (Aggregation.SystemVariable.isReferingToSystemVariable(reference)) {
+			if (SystemVariable.isReferingToSystemVariable(reference)) {
 				return reference;
 			}
 

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java

@@ -1395,7 +1395,7 @@ private Object renderFieldValue(AggregationOperationContext context) {
 				// implicit reference or explicit include?
 				if (value == null || Boolean.TRUE.equals(value)) {
 
-					if (Aggregation.SystemVariable.isReferingToSystemVariable(field.getTarget())) {
+					if (SystemVariable.isReferingToSystemVariable(field.getTarget())) {
 						return field.getTarget();
 					}
 

Diff for: spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SystemVariable.java

@@ -0,0 +1,116 @@
+/*
+ * Copyright 2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import org.springframework.lang.Nullable;
+
+/**
+ * Describes the system variables available in MongoDB aggregation framework pipeline expressions.
+ *
+ * @author Thomas Darimont
+ * @author Christoph Strobl
+ * @see <a href="https://docs.mongodb.com/manual/reference/aggregation-variables">Aggregation Variables</a>.
+ */
+public enum SystemVariable {
+
+	/**
+	 * Variable for the current datetime.
+	 * 
+	 * @since 4.0
+	 */
+	NOW,
+
+	/**
+	 * Variable for the current timestamp.
+	 * 
+	 * @since 4.0
+	 */
+	CLUSTER_TIME,
+
+	/**
+	 * Variable that references the root document.
+	 */
+	ROOT,
+
+	/**
+	 * Variable that references the start of the field path being processed.
+	 */
+	CURRENT,
+
+	/**
+	 * Variable that evaluates to a missing value.
+	 */
+	REMOVE,
+
+	/**
+	 * One of the allowed results of a {@literal $redact} expression
+	 * 
+	 * @since 4.0
+	 */
+	DESCEND,
+
+	/**
+	 * One of the allowed results of a {@literal $redact} expression
+	 * 
+	 * @since 4.0
+	 */
+	PRUNE,
+	/**
+	 * One of the allowed results of a {@literal $redact} expression
+	 * 
+	 * @since 4.0
+	 */
+	KEEP,
+
+	/**
+	 * A variable that stores the metadata results of an Atlas Search query.
+	 * 
+	 * @since 4.0
+	 */
+	SEARCH_META;
+
+	private static final String PREFIX = "$$";
+
+	/**
+	 * Return {@literal true} if the given {@code fieldRef} denotes a well-known system variable, {@literal false}
+	 * otherwise.
+	 *
+	 * @param fieldRef may be {@literal null}.
+	 * @return {@literal true} if the given field refers to a {@link SystemVariable}.
+	 */
+	public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
+
+		if (fieldRef == null || !fieldRef.startsWith(PREFIX) || fieldRef.length() <= 2) {
+			return false;
+		}
+
+		int indexOfFirstDot = fieldRef.indexOf('.');
+		String candidate = fieldRef.substring(2, indexOfFirstDot == -1 ? fieldRef.length() : indexOfFirstDot);
+
+		for (SystemVariable value : values()) {
+			if (value.name().equals(candidate)) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	@Override
+	public String toString() {
+		return PREFIX.concat(name());
+	}
+}

Diff for: spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java

@@ -19,7 +19,6 @@
 
 import org.bson.Document;
 import org.junit.jupiter.api.Test;
-import org.springframework.data.mongodb.core.aggregation.Aggregation.SystemVariable;
 import org.springframework.data.mongodb.core.aggregation.ObjectOperators.MergeObjects;
 
 /**