code: func BeforeCall(fn func(context.Context, proto.Message) error) Option {
  	return func(opts *options) {
  		opts.before = fn
  	}
  }
docs: // BeforeCall returns an option that allows intercepting a helm client rpc
  // before being sent OTA to tiller. The intercepting function should return
  // an error to indicate that the call should not proceed or nil otherwise.