FR: provide more detailed monitoring regarding temp table usage

Request:

• monitor the usage of the tmp space, trigger alerts when it crosses certain threshold
• find the query that's using the tmp space

Original description:

MariaDB currently only provides status counters for Created_tmp_tables and Created_tmp_disk_tables.

PostgreSQL for example provides more insight into this by tracking:

• total number of bytes written to temporary table files per database

See temp_bytes on https://www.postgresql.org/docs/9.2/monitoring-stats.html#PG-STAT-DATABASE-VIEW

• temporary file blocks read and written per statement

See temp_blks_read and temp_blks on https://www.postgresql.org/docs/9.0/pgstatstatements.html#PGSTATSTATEMENTS-COLUMNS