Support for additional audit config via env var (#26785)

* Add support for additional audit config via env var

server/channels/app/audit.go

@@ -4,10 +4,13 @@
 package app
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"net/http"
+	"os"
 	"os/user"
+	"strings"
 
 	"github.com/mattermost/mattermost/server/public/model"
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
@@ -132,6 +135,16 @@ func (s *Server) configureAudit(adt *audit.Audit, bAllowAdvancedLogging bool) er
 		return fmt.Errorf("invalid config for audit, %w", err)
 	}
 
+	// Append additional config from env var; any target name collisions will be overwritten.
+	additionalJSON := strings.TrimSpace(os.Getenv("MM_EXPERIMENTALAUDITSETTINGS_ADDITIONAL"))
+	if additionalJSON != "" {
+		cfgAdditional := make(mlog.LoggerConfiguration)
+		if err := json.Unmarshal([]byte(additionalJSON), &cfgAdditional); err != nil {
+			return fmt.Errorf("invalid additional config for audit, %w", err)
+		}
+		cfg.Append(cfgAdditional)
+	}
+
 	return adt.Configure(cfg)
 }