Creating a custom Slog level in Golang

package logger

import (
	"log/slog"
	"os"
	"time"
)

const (
	// Auto assigns `DEBUG-4` as label.
	LevelTrace = slog.Level(-8)
	// Auto assigns `ERROR+4` as label.
	LevelPanic = slog.Level(12)
)

var levels = map[slog.Leveler]string{
	LevelTrace: "TRACE",
	LevelPanic: "PANIC",
}

func Configure(severity string) {
	for level, label := range levels {
		if label == severity {
			severity = level.Level().String()

			break
		}
	}

	var level slog.Level
	if err := level.UnmarshalText([]byte(severity)); err != nil {
		level = slog.LevelError
	}

	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{
		Level: level,
		ReplaceAttr: func(groups []string, attr slog.Attr) slog.Attr {
			if attr.Key == slog.TimeKey {
				attr.Value = slog.StringValue(attr.Value.Time().Format(time.RFC3339))
			}

			if attr.Key == slog.MessageKey {
				attr.Key = "message"
			}

			if attr.Key == slog.LevelKey {
				level := attr.Value.Any().(slog.Level)
				label, ok := levels[level]
				if !ok {
					label = level.String()
				}

				attr.Value = slog.StringValue(label)
			}

			return attr
		},
	})))
}

$ go run -race main.go 
{"time":"2024-08-30T19:00:28+01:00","level":"TRACE","message":"trace msg"}
{"time":"2024-08-30T19:00:28+01:00","level":"DEBUG","message":"debug msg"}
{"time":"2024-08-30T19:00:28+01:00","level":"INFO","message":"info msg"}
{"time":"2024-08-30T19:00:28+01:00","level":"WARN","message":"warn msg"}
{"time":"2024-08-30T19:00:28+01:00","level":"ERROR","message":"error msg"}
{"time":"2024-08-30T19:00:28+01:00","level":"PANIC","message":"panic msg"}