set type of mysql_global_status_threads_* to gauge

[aliorouji]

We are interested in these metrics:

• mysql_global_status_threads_cached
• mysql_global_status_threads_connected
• mysql_global_status_threads_created
• mysql_global_status_threads_running

which current version of mysqld_exporter does not assign a metric type to them.

This PR will assign the correct metric type to them.

[SuperQ]

This needs a DCO sign-off. You can use git commit -s --amend to add it.