Allow better time precision in logs (#14)

Previously, the logs were sent with only second precision, which quickly
gets suboptimal when dealing with lots of log events happening at more
or less the same time.

This patch allows for milliseconds, as it sends the second value *with
decimals*.

handler.go

@@ -84,7 +84,7 @@ func (h *GraylogHandler) Handle(ctx context.Context, record slog.Record) error {
 		Host:     h.option.hostname,
 		Short:    short(&record),
 		Full:     strings.TrimSpace(record.Message),
-		TimeUnix: float64(record.Time.Unix()),
+		TimeUnix: float64(record.Time.UnixNano()) / 1e9,
 		Level:    LogLevels[record.Level],
 		Facility: h.option.Writer.Facility,
 		Extra:    extra,