grpc/auth/auth.go
func WithTokenFunc(k string, tokenFunc func(v string) string) *TokenCredentials {
	return &TokenCredentials{
		tokenFunc:    tokenFunc,
		tokenFuncKey: k,
	}
}
func WithTokenFunc(k string, tokenFunc func(v string) string) *TokenCredentials {
	return &TokenCredentials{
		tokenFunc:    tokenFunc,
		tokenFuncKey: k,
	}
}
// WithTokenFunc returns TokenCredentials that execute the tokenFunc on each request
// The value of v sent to the tokenFunk is the MD value of the supplied k