Adding ast aggregation operator

Author: singh-viikram

document-store/src/main/java/org/hypertrace/core/documentstore/expression/operators/AggregationOperator.java

@@ -11,4 +11,5 @@ public enum AggregationOperator {
   SUM,
   MIN,
   MAX,
+  LAST,
 }

document-store/src/main/java/org/hypertrace/core/documentstore/mongo/query/parser/MongoAggregateExpressionParser.java

@@ -5,6 +5,7 @@
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.COUNT;
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.DISTINCT;
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.DISTINCT_ARRAY;
+import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.LAST;
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.MAX;
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.MIN;
 import static org.hypertrace.core.documentstore.expression.operators.AggregationOperator.SUM;
@@ -30,6 +31,7 @@ final class MongoAggregateExpressionParser extends MongoSelectTypeExpressionPars
               put(MIN, "$min");
               put(MAX, "$max");
               put(COUNT, "$push");
+              put(LAST, "$last");
             }
           });