gRPC: how to override Channel Credentials Metadata with Call Credentials?

Question

C++. I have such code:

grpc::CompositeChannelCredentials(ssl_creds, StaticMetadataCredentialsPlugin::CreateCallCredentials("x-token", "12345"));

In some cases, I want override "x-token" metadata for specific call, but in both following cases, it doesn't override, it adds one more metadata with the same key:

ClientContext context;
context.AddMetadata("x-token", "overridden");
context.set_credentials(StaticMetadataCredentialsPlugin::CreateCallCredentials("x-token", "overridden"));

How to override metadata?