Nrpe-external-master relation is missing

[Connor Chamberlain]

The nrpe-external-master relation for this charm does not exist, which prevents charm-nrpe from rendering any MySQL-specific checks.

This can be reproduced by deploying mysql-innodb-cluster and adding nrpe as a subordinate. You will see that the juju-info relation is used by default. If you attempt to use the nrpe-external-master relation, you will see:

$ juju relate mysql-innodb-cluster:nrpe-external-master nrpe-container:nrpe-external-master
ERROR application "mysql-innodb-cluster" has no "nrpe-external-master" relation

Note that this relation does exist for the older mysql-percona-cluster charm.

Charm-nrpe is working as expected, and it will render any other Nagios checks based on its other relations. However, there is currently no way to render Nagios checks related to MySQL without building the charm locally or reverting to mysql-percona-cluster. The expected behavior here is that the relation I attempted above would succeed and result in several MySQL checks being rendered in /etc/nagios/nrpe.d/ on the mysql-innodb-cluster units.

This missing feature impacts the efficacy of Nagios as a monitoring tool for mysql-innodb-cluster but does not impact the functionality of mysql-innodb-cluster itself.

[Przemyslaw Lal (przemeklal)]

Please note that this was implemented over a year ago but denied:
https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/818401