MM-48984: Add missing timeout while creating a connection (#21847)

If a timeout is missing, this goroutine waits indefinitely trying
to get a connection. Leading to a goroutine accumulation in a scenario
where the DB is somehow not release connections.

https://mattermost.atlassian.net/browse/MM-48984

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/plugin_db_driver.go

@@ -8,6 +8,7 @@ import (
 	"database/sql"
 	"database/sql/driver"
 	"sync"
+	"time"
 
 	"github.com/mattermost/mattermost-server/v6/model"
 	"github.com/mattermost/mattermost-server/v6/plugin"
@@ -44,7 +45,10 @@ func (d *DriverImpl) Conn(isMaster bool) (string, error) {
 	if !isMaster {
 		dbFunc = d.s.Platform().Store.GetInternalReplicaDB
 	}
-	conn, err := dbFunc().Conn(context.Background())
+	timeout := time.Duration(*d.s.Config().SqlSettings.QueryTimeout) * time.Second
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+	conn, err := dbFunc().Conn(ctx)
 	if err != nil {
 		return "", err
 	}

app/plugin_db_driver_test.go

@@ -0,0 +1,21 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestConnCreateTimeout(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	*th.App.Config().SqlSettings.QueryTimeout = 0
+
+	d := NewDriverImpl(th.Server)
+	_, err := d.Conn(true)
+	require.Error(t, err)
+}