Add additional events statements metrics
Add additional events statements metrics

* sort merge passes - lots of merge passes can indicate too small a
  sort_buffer_size
* sort rows - total number of rows sorted, useful for finding expensive
  statements
* no index used - statements that cause full table scans

prometheus/mysqld_exporter,roman-vynar/mysqld_exporter,growbots/mysqld_exporter,roman-vynar/mysqld_exporter,prometheus/mysqld_exporter