Add support for Percona userstats output.
Add support for Percona userstats output.

This commit adds support for dumping Percona userstats. These metrics are per
user counters are are exported as such to enable detection of resets by
Prometheus.

The new functionality, being limited to Percona, is enabled by invoking with
-collect.info_schema.userstats=true

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