[MM-18638] Send mlog console output to stderr (#12366)

* Send server logs to stderr

* Update comment

mlog/default.go

@@ -6,9 +6,10 @@ package mlog
 import (
 	"encoding/json"
 	"fmt"
+	"os"
 )
 
-// defaultLog manually encodes the log to STDOUT, providing a basic, default logging implementation
+// defaultLog manually encodes the log to STDERR, providing a basic, default logging implementation
 // before mlog is fully configured.
 func defaultLog(level, msg string, fields ...Field) {
 	log := struct {
@@ -22,9 +23,9 @@ func defaultLog(level, msg string, fields ...Field) {
 	}
 
 	if b, err := json.Marshal(log); err != nil {
-		fmt.Printf(`{"level":"error","msg":"failed to encode log message"}%s`, "\n")
+		fmt.Fprintf(os.Stderr, `{"level":"error","msg":"failed to encode log message"}%s`, "\n")
 	} else {
-		fmt.Printf("%s\n", b)
+		fmt.Fprintf(os.Stderr, "%s\n", b)
 	}
 }
 

mlog/log.go

@@ -86,7 +86,7 @@ func NewLogger(config *LoggerConfiguration) *Logger {
 	}
 
 	if config.EnableConsole {
-		writer := zapcore.Lock(os.Stdout)
+		writer := zapcore.Lock(os.Stderr)
 		core := zapcore.NewCore(makeEncoder(config.ConsoleJson), writer, logger.consoleLevel)
 		cores = append(cores, core)
 	}