code: func (client *GRPCClient) Certificate() tls.Certificate {
  	cert := tls.Certificate{}
  	if client.tlsConfig != nil && len(client.tlsConfig.Certificates) > 0 {
  		cert = client.tlsConfig.Certificates[0]
  	}
  	return cert
  }
docs: // Certificate returns the tls.Certificate used to make TLS connections
  // when client certificates are required by the server