The following global status variables where added:
* binlog_group_commit_reason_count
* binlog_group_commit_reason_usec
* binlog_group_commit_reason_transaction
* binlog_group_commit_reason_immediate

binlog_group_commit_reason_count corresponds to group commits made by
virtue of the binlog_commit_wait_count variable.

binlog_group_commit_reason_usec corresponds to the binlog_commit_wait_usec
variable.

binlog_group_commit_reason_transaction is a result of ordered
transaction that need to occur in the same order on the slave and can't
be parallelised.

binlog_group_commit_reason_immediate is caused to prevent stalls with
row locks as described in log.cc:binlog_report_wait_for. This immediate
count is also counted a second time in binlog_group_commit_reason_transaction.

Overall binlog_group_commits = binlog_group_commit_reason_count +
binlog_group_commit_reason_usec + binlog_group_commit_reason_transaction

This work was funded thanks to Open Source Developers Club Australia.