code: public ExecutionResult addEvent(HystrixEventType eventType) {
          return new ExecutionResult(eventCounts.plus(eventType), startTimestamp, executionLatency,
                  userThreadLatency, failedExecutionException, executionException,
                  executionOccurred, isExecutedInThread, collapserKey);
      }
docs: Creates a new ExecutionResult by adding the defined 'event' to the ones on the current instance.
  
  @param eventType event to add
  @return new {@link ExecutionResult} with event added