code: def _init_channel(self):
          
          host = self._get_host()
          port = self._get_grpc_port()
  
          if 'TLS_PEM_FILE' in os.environ:
              with open(os.environ['TLS_PEM_FILE'], mode='rb') as f:  # b is important -> binary
                  file_content = f.read()
              credentials = grpc.ssl_channel_credentials(root_certificates=file_content)
          else:
              credentials = grpc.ssl_channel_credentials()
  
          self._channel = grpc.secure_channel(host + ":" + port, credentials=credentials)
          self._init_health_checker()
docs: build the grpc channel used for both publisher and subscriber
          :return: None