fix(aggregate): allow passing verbosity to Aggregate.prototype.explain()

Fix #11144

Author: vkarpov15

index.d.ts

@@ -2817,6 +2817,7 @@ declare module 'mongoose' {
 
     /** Execute the aggregation with explain */
     explain(callback?: Callback): Promise<any>;
+    explain(verbosity?: string, callback?: Callback): Promise<any>;
 
     /** Combines multiple aggregation pipelines. */
     facet(options: PipelineStage.Facet['$facet']): this;

lib/aggregate.js

@@ -710,12 +710,17 @@ Aggregate.prototype.redact = function(expression, thenExpr, elseExpr) {
  *
  *     Model.aggregate(..).explain(callback)
  *
+ * @param {String} verbosity
  * @param {Function} callback
  * @return {Promise}
  */
 
-Aggregate.prototype.explain = function(callback) {
+Aggregate.prototype.explain = function(verbosity, callback) {
   const model = this._model;
+  if (typeof verbosity === 'function') {
+    callback = verbosity;
+    verbosity = null;
+  }
 
   return promiseOrCallback(callback, cb => {
     if (!this._pipeline.length) {
@@ -733,24 +738,34 @@ Aggregate.prototype.explain = function(callback) {
         });
       }
 
-      this.options.explain = true;
-
       model.collection.aggregate(this._pipeline, this.options, (error, cursor) => {
         if (error != null) {
           const _opts = { error: error };
           return model.hooks.execPost('aggregate', this, [null], _opts, error => {
             cb(error);
           });
         }
-        cursor.explain((error, result) => {
-          const _opts = { error: error };
-          return model.hooks.execPost('aggregate', this, [result], _opts, error => {
-            if (error) {
-              return cb(error);
-            }
-            return cb(null, result);
+        if (verbosity != null) {
+          cursor.explain(verbosity, (error, result) => {
+            const _opts = { error: error };
+            return model.hooks.execPost('aggregate', this, [result], _opts, error => {
+              if (error) {
+                return cb(error);
+              }
+              return cb(null, result);
+            });
           });
-        });
+        } else {
+          cursor.explain((error, result) => {
+            const _opts = { error: error };
+            return model.hooks.execPost('aggregate', this, [result], _opts, error => {
+              if (error) {
+                return cb(error);
+              }
+              return cb(null, result);
+            });
+          });
+        }
       });
     });
   }, model.events);