manual revert of #8901
diff --git a/test/cpp/interop/interop_server.cc b/test/cpp/interop/interop_server.cc
index 67456ce..956840b 100644
--- a/test/cpp/interop/interop_server.cc
+++ b/test/cpp/interop/interop_server.cc
@@ -56,6 +56,7 @@
#include "test/cpp/util/test_config.h"
DEFINE_bool(use_tls, false, "Whether to use tls.");
+DEFINE_string(custom_credentials_type, "", "User provided credentials type.");
DEFINE_int32(port, 0, "Server port.");
DEFINE_int32(max_send_message_size, -1, "The maximum send message size.");