mysql-table-cache: Add table_open_cache_overflows over uptime as metric

[u-j-moser]

Describe the solution you'd like

Add table_open_cache_overflows and table_open_cache_overflows per second to the metrics of mysql-table-cache plugin

Additional context

If a MySQL/MariaDB instance has a lot of table cache overflows this can also cause significant performance degradation because tables get opened and closed over and over again causing additional checkpoints because they first need to be cleaned up before closing them and opening is also an expensive task. That's why table_open_cache exists.

[markuslf]

Do you have any alerting threshold recommendations / best practises, or should we just report the values in perfdata (because it's too specific per server)?

[u-j-moser]

Hi Markus, this value should be 0. Any Value > 0 indicates a table cache being too small.