code: func GetSigner(c context.Context) signing.Signer {
  	if cfg := getConfig(c); cfg != nil {
  		return cfg.Signer
  	}
  	return nil
  }
docs: // GetSigner returns the signing.Signer instance representing the service.
  //
  // It is injected into the context as part of Config. It can be used to grab
  // a service account the service is running as or its current public
  // certificates.
  //
  // Returns nil if the signer is not available.