[Metricbeat] Revisit MySQL module

Today MySQL module collects many relevant metrics, however some are missing. E.g.

* innodb rows {deleted, updated, read, inserted}
* slow queries
* row locks
* CPU time spent by MySQL process
* cache stats (`SHOW STATUS LIKE "qcache%"`), including cache size.
* buffer pool size (innodb_buffer_pool_size)
* connection errors (`SHOW GLOBAL STATUS LIKE 'Connection_errors%';`)
* database keyword to use for filtering

Start in the Metricbeat MySQL module and compare its existing collection with the requested InnoDB, slow-query, row-lock, CPU, cache, buffer-pool, connection-error, and database-filtering metrics. Done means the listed MySQL values are collected and the database keyword can be used for filtering; validate the relevant SHOW STATUS queries and configuration value.

Written by the indexing model from the issue text.