implement io.WriterTo on byteCountReadCloser

hlog/internal/mutil/body.go

@@ -11,6 +11,7 @@ type byteCountReadCloser struct {
 }
 
 var _ io.ReadCloser = (*byteCountReadCloser)(nil)
+var _ io.WriterTo = (*byteCountReadCloser)(nil)
 
 func NewByteCountReadCloser(rc io.ReadCloser) *byteCountReadCloser {
 	return &byteCountReadCloser{
@@ -28,6 +29,12 @@ func (b *byteCountReadCloser) Close() error {
 	return b.rc.Close()
 }
 
+func (b *byteCountReadCloser) WriteTo(w io.Writer) (int64, error) {
+	n, err := io.Copy(w, b.rc)
+	atomic.AddInt64(&b.read, n)
+	return n, err
+}
+
 func (b *byteCountReadCloser) BytesRead() int64 {
 	return b.read
 }