logtail: fix Logger.Write return result

io.Writer says you need to write completely on err=nil. (the result
int should be the same as the input buffer length)

We weren't doing that. We used to, but at some point the verbose
filtering was modifying buf before the final return of len(buf).

We've been getting lucky probably, that callers haven't looked at our
results and turned us into a short write error.

Updates #cleanup
Updates tailscale/corp#15664

Change-Id: I01e765ba35b86b759819e38e0072eceb9d10d75c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

logtail/logtail.go

@@ -735,6 +735,8 @@ func (l *Logger) Write(buf []byte) (int, error) {
 	if len(buf) == 0 {
 		return 0, nil
 	}
+	inLen := len(buf) // length as provided to us, before modifications to downstream writers
+
 	level, buf := parseAndRemoveLogLevel(buf)
 	if l.stderr != nil && l.stderr != io.Discard && int64(level) <= atomic.LoadInt64(&l.stderrLevel) {
 		if buf[len(buf)-1] == '\n' {
@@ -752,7 +754,7 @@ func (l *Logger) Write(buf []byte) (int, error) {
 
 	b := l.encodeLocked(buf, level)
 	_, err := l.sendLocked(b)
-	return len(buf), err
+	return inLen, err
 }
 
 var (