client/v3/watch.go: use fmt for metadata printing

Signed-off-by: Mohamed Awnallah <[email protected]>

client/v3/watch.go

@@ -1037,7 +1037,7 @@ func (pr *progressRequest) toPB() *pb.WatchRequest {
 
 func streamKeyFromCtx(ctx context.Context) string {
 	if md, ok := metadata.FromOutgoingContext(ctx); ok {
-		return fmt.Sprintf("%+v", md)
+		return fmt.Sprintf("%+v", map[string][]string(md))
 	}
 	return ""
 }