log: add Logger.Writer method

Writer method returns the underlying io.Writer used by the given Logger
object.

Fixes #28327

Change-Id: I6347913d8be8d3222b98967b136cf03d00f446d1
GitHub-Last-Rev: 2db0c5c7416436f99852592d2f258ca93da96712
GitHub-Pull-Request: golang/go#28399
Reviewed-on: https://go-review.googlesource.com/c/144757Reviewed-by: Rob Pike <r@golang.org>

src/log/log.go

@@ -254,6 +254,13 @@ func (l *Logger) SetPrefix(prefix string) {
 	l.prefix = prefix
 }
 
+// Writer returns the output destination for the logger.
+func (l *Logger) Writer() io.Writer {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	return l.out
+}
+
 // SetOutput sets the output destination for the standard logger.
 func SetOutput(w io.Writer) {
 	std.mu.Lock()