Issue:
In SQL 2012, stored procedures that call any other stored procedures or 

extended stored procedures that return result sets should specify WITH RESULT SETS

SQL Agent's stored procedure sp_help_job 

- returns single resultset if no parameters are passed to stored procedure. 

- returns 3 resultsets if job_id or job_name is specified. MSDN link

When RESULTSET is not described, Openrowset on msdb.dbo.sp_help_job throws following error

Msg 11520, Level 16, State 1, Procedure sp_describe_first_result_set, Line 1
The metadata could not be determined because statement 'EXECUTE master.dbo.xp_sqlagent_is_starting @retval OUTPUT' in procedure 'sp_is_sqlagent_starting' invokes an extended stored procedure.


Workaround:
Create a wrapper stored procedure that calls sp_help_job WITH RESULT SET() 

and use the wrapper stored procedure in OPENROWSET()

Sample Code:
T-SQL script can downloaded from here

Known issues:
1) OPENROWSET returns only the first rowset. 
2) sp_describe_first_result_set returns only metadata for first resultset. 

If you have any issues, please report to SQL Server team.