Add support for metrics collection (mysql-root interface)

[Alvaro Uria]

* charm-telegraf [1] supports a mysql plugin through the mysql-root interface.
* charm-percona-cluster has a "db-admin" endpoint that uses the mysql-root interface to configure a user for the telegraf mysql plugin
* This charm (charm-mysql-innodb-cluster) should support a similar endpoint (db-admin) so telegraf can collect metrics from each mysql server.

Note: the mysql-root interface collects the "host" attribute (see [2]), which in the case of charm-percona-cluster defaults to the VIP configured (so all 3 telegraf subordinates collect metrics from a single mysql instance). For charm-mysql-innodb-cluster, the "host" attribute should be each of the "private-address" values, so telegraf will collect information about each of the instances (1x RW mode, 2x RO mode).

The Juju configuration mode would be:
# makes telegraf a subordinate, with default plugins enabled
juju relate mysql-innodb-cluster telegraf:juju-info
# enables the mysql plugin
juju relate mysql-innodb-cluster:db-admin telegraf:mysql
# exposes the 0.0.0.0:9103/metrics service in telegraf
juju relate prometheus:target telegraf
# unrelated to this bug: share a mysql dashboard with Grafana,
# so the collected metrics by prometheus are displayed
juju relate grafana:dashboards telegraf

1. https://launchpad.net/charm-telegraf/
2. https://gist.github.com/auria/c9f23772ce2ddb5797d82204d9fabd59

[Liam Young (gnuoy)]

Sounds like a good addition to the charm, thanks. As a side-note I think you were suggesting using private-address to illustrate your point, but the implementer should remember that private-address should not be used as its deprecated and unreliable.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/817081

[Robert Gildein (rgildein)]

After discussing in the proposed change, we decided to use the new
interface instead of `mysql-root`, because the `mysql-root` was meant
to be used in different type of situation.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/817081
Committed: https://opendev.org/openstack/charm-mysql-innodb-cluster/commit/d8d1dc0527046cc3dd41f4394575f14e9675f230
Submitter: "Zuul (22348)"
Branch: master

commit d8d1dc0527046cc3dd41f4394575f14e9675f230
Author: Robert Gildein <email address hidden>
Date: Tue Nov 9 19:31:05 2021 +0100

    Add db-monitor relation w/ mysql-monitor interface

    The db-monitor relation provides to relate charm-telegraf via the
    mysql-monitor interface, which provides mysql plugin for collecting
    metrics.

    Closes-Bug: #1938865
    Change-Id: I2d0351263ecf3e79861512376f41974f2f7fc304

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/focal
Review: https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/875616

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Seyeong Kim <email address hidden>" on branch: stable/focal
Review: https://review.opendev.org/c/openstack/charm-mysql-innodb-cluster/+/875616
Reason: deprecated stable/focal