add beforeConnect handler

Allow mutating the tedious connection by passing it to a beforeConnect handler, passed in the configuration object.
This for example allows to tap into the tedious.debug methods and collect metrics or monitor connect, end or other events on the connection.

README.md

@@ -405,6 +405,13 @@ Default driver, actively maintained and production ready. Platform independent,
 
 **Extra options:**
 
+- **beforeConnect(conn)** - Function, which is invoked before opening the connection. The parameter `conn` is the configured tedious `Connection`. It can be used for attaching event handlers like in this example:
+```js
+require('mssql').connect(...config, beforeConnect: conn => {
+  conn.once('connect', err => { err ? console.error(err) : console.log('mssql connected')})
+  conn.once('end', err => { err ? console.error(err) : console.log('mssql disconnected')})
+}})
+```
 - **options.instanceName** - The instance name to connect to. The SQL Server Browser service must be running on the database server, and UDP port 1434 on the database server must be reachable.
 - **options.useUTC** - A boolean determining whether or not use UTC time for values without time zone offset (default: `true`).
 - **options.encrypt** - A boolean determining whether or not the connection will be encrypted (default: `false`).
@@ -420,6 +427,7 @@ More information about Tedious specific options: http://tediousjs.github.io/tedi
 
 **Extra options:**
 
+- **beforeConnect(conn)** - Function, which is invoked before opening the connection. The parameter `conn` is the connection configuration, that can be modified to pass extra parameters to the driver's `open()` method.
 - **connectionString** - Connection string (default: see below).
 - **options.instanceName** - The instance name to connect to. The SQL Server Browser service must be running on the database server, and UDP port 1444 on the database server must be reachable.
 - **options.trustedConnection** - Use Windows Authentication (default: `false`).

lib/msnodesqlv8.js

@@ -183,6 +183,10 @@ class ConnectionPool extends base.ConnectionPool {
       debug('pool(%d): connection #%d created', IDS.get(this), connedtionId)
       debug('connection(%d): establishing', connedtionId)
 
+      if (typeof this.config.beforeConnect === 'function') {
+        this.config.beforeConnect(cfg)
+      }
+
       msnodesql.open(cfg, (err, tds) => {
         if (err) {
           err = new base.ConnectionError(err)

lib/tedious.js

@@ -258,6 +258,9 @@ class ConnectionPool extends base.ConnectionPool {
       if (this.config.debug) {
         tedious.on('debug', this.emit.bind(this, 'debug', tedious))
       }
+      if (typeof this.config.beforeConnect === 'function') {
+        this.config.beforeConnect(tedious)
+      }
     })
   }