prometheus_exporters: add SLAVE MONITOR MySQL grant

Fixes the following error logged to the journal:

> prometheus-mysqld-exporter[2247]: ts=2023-12-24T13:16:45.938Z
> caller=exporter.go:173 level=error msg="Error from scraper"
> scraper=slave_status target=localhost:3306 err="Error 1227 (42000):
> Access denied; you need (at least one of) the SLAVE MONITOR privilege(s)
> for this operation"

[1] https://github.com/prometheus/mysqld_exporter/issues/640

roles/prometheus_exporters/tasks/main.yml

@@ -25,7 +25,7 @@
   mysql_user:
     name: '{{ prometheus_mysqld_user }}'
     password: '{{ vault_monitoring_mysql_password }}'
-    priv: "*.*:PROCESS,REPLICATION CLIENT"
+    priv: "*.*:PROCESS,REPLICATION CLIENT,SLAVE MONITOR"
     state: present
     resource_limits:
       MAX_USER_CONNECTIONS: 3