Revert "fix(promslog): always use UTC for time (#735)"

This reverts commit 145b50a.

See discussion on Slack. We should first decide which log format we
want before changing it from what is already released in
prometheus/prometheus.

promslog/slog.go

@@ -68,16 +68,13 @@ var (
 
 		return a
 	}
-	defaultReplaceAttrFunc = func(groups []string, a slog.Attr) slog.Attr {
-		key := a.Key
-		switch key {
-		case slog.TimeKey:
-			t := a.Value.Time()
-			a.Value = slog.TimeValue(t.UTC())
-		case slog.SourceKey:
-			src, _ := a.Value.Any().(*slog.Source)
+	truncateSourceAttrFunc = func(groups []string, a slog.Attr) slog.Attr {
+		if a.Key != slog.SourceKey {
+			return a
+		}
+
+		if src, ok := a.Value.Any().(*slog.Source); ok {
 			a.Value = slog.StringValue(filepath.Base(src.File) + ":" + strconv.Itoa(src.Line))
-		default:
 		}
 
 		return a
@@ -181,7 +178,7 @@ func New(config *Config) *slog.Logger {
 	logHandlerOpts := &slog.HandlerOptions{
 		Level:       config.Level.lvl,
 		AddSource:   true,
-		ReplaceAttr: defaultReplaceAttrFunc,
+		ReplaceAttr: truncateSourceAttrFunc,
 	}
 
 	if config.Style == GoKitStyle {