code: public static long getOrCreateFlowExecutionId(FlowSpec spec) {
      long flowExecutionId;
      if (spec.getConfig().hasPath(ConfigurationKeys.FLOW_EXECUTION_ID_KEY)) {
        flowExecutionId = spec.getConfig().getLong(ConfigurationKeys.FLOW_EXECUTION_ID_KEY);
      } else {
        flowExecutionId = System.currentTimeMillis();
      }
      return flowExecutionId;
    }
docs: A FlowSpec contains a FlowExecutionId if it is a runOnce flow.
  Refer {@link FlowConfigResourceLocalHandler#createFlowSpecForConfig} for details.
  @param spec flow spec
  @return flow execution id