internal/telemetry: obey the onlyErrors flag in the log writer

has no impact because there are no use cases that don't set it to true right now

Change-Id: I2bc485226078c710bdc36397b96755cdce82d9cc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/212242
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
diff --git a/internal/telemetry/export/log.go b/internal/telemetry/export/log.go
index d840e53..a63ad47 100644
--- a/internal/telemetry/export/log.go
+++ b/internal/telemetry/export/log.go
@@ -28,8 +28,7 @@
 func (w *logWriter) StartSpan(context.Context, *telemetry.Span)  {}
 func (w *logWriter) FinishSpan(context.Context, *telemetry.Span) {}
 func (w *logWriter) Log(ctx context.Context, event telemetry.Event) {
-	if event.Error == nil {
-		// we only log errors by default
+	if w.onlyErrors && event.Error == nil {
 		return
 	}
 	fmt.Fprintf(w.writer, "%v\n", event)