Add metrics for events statements by digest
Add metrics for events statements by digest

Read metrics from performance_schema.events_statements_summary_by_digest
* Limit to the top digests by count (default to 250)
* Disabled by default becuase it can produce a large number of metrics

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