MSSQL - Skip isolationLevel query, enable tedious debugging (#7130)

* MSSQL - Skip isolationLevel query, enable tedious debugging

* Added changelog entry, updated docs

changelog.md

 # Future
+- [CHANGED] `setIsolationLevelQuery` to skip under MSSQL dialect, added debug listener for tedious [#7130](https://github.com/sequelize/sequelize/pull/7130)
 - [FIXED] `removeColumn` method to support dropping primaryKey column (MSSQL) [#7081](https://github.com/sequelize/sequelize/pull/7081)
 - [ADDED] Filtered Indexes support for SQL Server [#7016](https://github.com/sequelize/sequelize/issues/7016)
 - [FIXED] Set `timestamps` and `paranoid` options from through model on `belongsToMany` association

docs/docs/transactions.md

@@ -134,6 +134,8 @@ return sequelize.transaction({
   });
 ```
 
+Note: The SET ISOLATION LEVEL queries are not logged in case of MSSQL as the specified isolationLevel is passed directly to tedious
+
 # Unmanaged transaction (then-callback)
 Unmanaged transactions force you to manually rollback or commit the transaction. If you don't do that, the transaction will hang until it times out. To start an unmanaged transaction, call `sequelize.transaction()` without a callback (you can still pass an options object) and call `then` on the returned promise. Notice that `commit()` and `rollback()` returns a promise.
 

lib/dialects/mssql/connection-manager.js

@@ -5,6 +5,7 @@ const ResourceLock = require('./resource-lock');
 const Promise = require('../../promise');
 const Utils = require('../../utils');
 const debug = Utils.getLogger().debugContext('connection:mssql');
+const debugTedious = Utils.getLogger().debugContext('connection:mssql:tedious');
 const sequelizeErrors = require('../../errors');
 const parserStore = require('../parserStore')('mssql');
 const _ = require('lodash');
@@ -111,6 +112,10 @@ class ConnectionManager extends AbstractConnectionManager {
         }
       });
       
+      if (config.dialectOptions.debug) {
+        connection.on('debug', debugTedious);        
+      }
+
       if (config.pool.handleDisconnects) {
         connection.on('error', err => {
           switch (err.code) {

lib/dialects/mssql/query-generator.js

@@ -730,11 +730,7 @@ var QueryGenerator = {
   },
 
   setIsolationLevelQuery(value, options) {
-    if (options.parent) {
-      return;
-    }
     
-    return 'SET TRANSACTION ISOLATION LEVEL ' + value + ';';
   },
 
   generateTransactionId() {