code: def update_webhook_metadata(self, scaling_group, policy, webhook, metadata):
          
          if not isinstance(webhook, AutoScaleWebhook):
              webhook = self.get_webhook(scaling_group, policy, webhook)
          curr_meta = webhook.metadata or {}
          curr_meta.update(metadata)
          return self.update_webhook(scaling_group, policy, webhook,
                  metadata=curr_meta)
docs: Adds the given metadata dict to the existing metadata for the specified
          webhook.