for trace-level logging in console format (as opposed to logfmt), print the trace as quoted string

so we can easily see the exact bytes on the wire, instead of having \n's
expanded as newlines. much easier to read. we had this in the past, but it must
have been lost in a refactor.

mlog/log.go

@@ -575,7 +575,11 @@ func (h *handler) write(l slog.Level, r slog.Record) error {
 			}
 		}
 
-		fmt.Fprint(eb, LevelStrings[r.Level], ": ", r.Message)
+		msg := r.Message
+		if r.Level <= LevelTrace {
+			msg = fmt.Sprintf("%q", msg)
+		}
+		fmt.Fprint(eb, LevelStrings[r.Level], ": ", msg)
 		n := 0
 		r.Attrs(func(a slog.Attr) bool {
 			if n == 0 && a.Key == "err" && h.Group == "" {