Payload fails to start after charm upgrade, missing /run/mysql directory

[Frode Nordahl]

After upgrade to version 15 of the charm the mysql-router fails to start due to a missing `mysql` directory in `/run`:

root@juju-de4186-6-lxd-0:/var/lib/mysql/aodh-mysql-router/log# stat /etc/systemd/system/aodh-mysql-router.service
  File: /etc/systemd/system/aodh-mysql-router.service
  Size: 356 Blocks: 8 IO Block: 4096 regular file
Device: fd01h/64769d Inode: 17049834 Links: 1
Access: (0755/-rwxr-xr-x) Uid: ( 0/ root) Gid: ( 124/ mysql)
Access: 2022-03-30 05:28:10.350289071 +0000
Modify: 2022-03-30 05:28:10.342288979 +0000
Change: 2022-03-30 05:28:10.342288979 +0000
 Birth: -
root@juju-de4186-6-lxd-0:/var/lib/mysql/aodh-mysql-router/log# systemctl status aodh-mysql-router
● aodh-mysql-router.service - MySQL Router
     Loaded: loaded (/etc/systemd/system/aodh-mysql-router.service; enabled; vendor preset: enabled)
     Active: active (exited) since Wed 2022-03-30 05:28:06 UTC; 3min 34s ago
   Main PID: 875138 (code=exited, status=0/SUCCESS)
      Tasks: 0 (limit: 314572)
     Memory: 8.0K
     CGroup: /system.slice/aodh-mysql-router.service

Mar 30 05:28:06 juju-de4186-6-lxd-0 systemd[1]: Starting MySQL Router...
Mar 30 05:28:06 juju-de4186-6-lxd-0 systemd[1]: Started MySQL Router.
Mar 30 05:28:06 juju-de4186-6-lxd-0 sudo[875140]: root : TTY=unknown ; PWD=/ ; USER=root ; ENV=ROUTER_PID=/var/lib/mysql/aodh-mysql-router/mysqlrouter.pid ; COMMAND=/usr/bin/mysqlrouter -c /var/lib/mysql/aodh-mysql-router/mysqlrouter.conf --user=mysql
Mar 30 05:28:06 juju-de4186-6-lxd-0 sudo[875140]: pam_unix(sudo:session): session opened for user root by (uid=0)
Mar 30 05:28:06 juju-de4186-6-lxd-0 start.sh[875146]: Error: Failed writing PID to '/run/mysql/mysqlrouter-aodh-mysql-router.pid': No such file or directory
Mar 30 05:28:06 juju-de4186-6-lxd-0 sudo[875140]: pam_unix(sudo:session): session closed for user root