As noted in
MDEV-10509, MDEV-6812 introduced server-wide status variables for monitoring ALTER TABLE...ALGORITHM=INPLACE progress. That feels wrong, because there can be multiple ALTER TABLE being executed concurrently.
I think that we should report progress via the native MariaDB interface instead of spamming the error log.
In 10.2, the InnoDB code to report progress via a similar interface already exists. It only needs to be tied to the proper interface. For 10.1, I think that we can backport the progress reporting changes from 10.2.