Monitor progress of ALTER TABLE ... ADD INDEX, ALGORITHM=INPLACE for InnoDB

When adding an index to an InnoDB table using algorithm=inplace, it is not possible to monitor the progress of the merge sort operation, which can take a great deal of time.

It would be nice if the "Progress" column of SHOW PROCESSLIST was updated with some information about how far along the merge sort has progressed. It might also be possible or desirable to make some progress information available by other means.