public void addOutputData(Attribute attribute) {
        if (outputData == null) {
            outputData = new ArrayList<Attribute>();
        }
        if (!outputData.contains(attribute)) {
            outputData.add(attribute);
        }
    }
Universal method to add data to MetaStream event.
Will make sure event will be added to corresponding array by
initializing them accordingly.

@param attribute attribute
@return attribute position
modules/siddhi-core/src/main/java/io/siddhi/core/event/stream/MetaStreamEvent.java