MM-23222 mirror audit logs to file (#14062)

* MM-23222 add file target (with rotation) to audit

* MM-23222 mirror syslog audits to local filesystem

    * provides config options for file name, max size, max age

    * rotates files based on max size and max age; delete as needed based on max backups

* include cluster id in log records

* sort meta data fields

audit/file.go

@@ -0,0 +1,34 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package audit
+
+import (
+	"os"
+
+	"github.com/wiggin77/logr"
+	"github.com/wiggin77/logr/target"
+)
+
+type FileOptions target.FileOptions
+
+// NewFileTarget creates a target capable of outputting log records to a rotated file.
+func NewFileTarget(filter logr.Filter, formatter logr.Formatter, opts FileOptions, maxQSize int) (*target.File, error) {
+	fopts := target.FileOptions(opts)
+	err := checkFileWritable(fopts.Filename)
+	if err != nil {
+		return nil, err
+	}
+	target := target.NewFileTarget(filter, formatter, fopts, maxQSize)
+	return target, nil
+}
+
+func checkFileWritable(filename string) error {
+	// try opening/creating the file for writing
+	file, err := os.OpenFile(filename, os.O_RDWR|os.O_APPEND|os.O_CREATE, 0600)
+	if err != nil {
+		return err
+	}
+	file.Close()
+	return nil
+}