appengine.go
func WithAPICallFunc(ctx context.Context, f APICallFunc) context.Context {
	return internal.WithCallOverride(ctx, internal.CallOverrideFunc(f))
}
func WithAPICallFunc(ctx context.Context, f APICallFunc) context.Context {
	return internal.WithCallOverride(ctx, internal.CallOverrideFunc(f))
}
// WithAPICallFunc returns a copy of the parent context
// that will cause API calls to invoke f instead of their normal operation.
//
// This is intended for advanced users only.