feat(aggregation): adds support for comment in aggregation command

lib/collection.js

@@ -2426,6 +2426,7 @@ function decorateWithCollation(command, self, options) {
  * @param {boolean} [options.promoteValues=true] Promotes BSON values to native types where possible, set to false to only receive wrapper types.
  * @param {boolean} [options.promoteBuffers=false] Promotes Binary BSON values to native Node Buffers.
  * @param {object} [options.collation=null] Specify collation (MongoDB 3.4 or higher) settings for update operation (see 3.4 documentation for available fields).
+ * @param {string} [options.comment] Add a comment to an aggregation command
  * @param {ClientSession} [options.session] optional session to use for this operation
  * @param {Collection~resultCallback} callback The command result callback
  * @return {(null|AggregationCursor)}
@@ -2515,6 +2516,8 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
   // If explain has been specified add it
   if (options.explain) command.explain = options.explain;
 
+  if (typeof options.comment === 'string') command.comment = options.comment;
+
   // Validate that cursor options is valid
   if (options.cursor != null && typeof options.cursor !== 'object') {
     throw toError('cursor options must be an object');

test/functional/aggregation_tests.js

@@ -1081,6 +1081,45 @@ describe('Aggregation', function() {
     }
   );
 
+  it('should pass a comment down via the aggregation command', {
+    metadata: {
+      requires: {
+        mongodb: '>=3.5.0',
+        topology: 'single',
+        node: '>=4.8.5'
+      }
+    },
+    test: function(done) {
+      const client = this.configuration.newClient({ w: 1 }, { poolSize: 1 });
+      const databaseName = this.configuration.db;
+
+      const comment = 'Darmok and Jalad at Tanagra';
+
+      client.connect(function(err, client) {
+        expect(err).to.be.null;
+
+        const db = client.db(databaseName);
+        const collection = db.collection('testingPassingDownTheAggregationCommand');
+
+        const command = db.command;
+
+        db.command = function(c) {
+          expect(c).to.be.an('object');
+          expect(c.comment)
+            .to.be.a('string')
+            .and.to.equal('comment');
+          command.apply(db, Array.prototype.slice.call(arguments, 0));
+        };
+
+        collection.aggregate([{ $project: { _id: 1 } }], { comment }, function(err, r) {
+          expect(err).to.be.null;
+          expect(r).to.not.be.null;
+          done();
+        });
+      });
+    }
+  });
+
   /**
    * Correctly call the aggregation framework to return a cursor with batchSize 1 and get the first result using next
    *