Hi Arun,
Thanks for a quick response, yups that's correct..! 
Solution : metadatamap2.get("KEY_WORKFLOW_STAGE", String.class);
I also found this same method from JcrValueMap class just sometime after posting this question, tried it & it worked.
Also, my import is - 'import com.adobe.granite.workflow.metadata.MetaDataMap;' and it works just fine.
FYI, I'm using AEM 6.4.
Regards,