update-status interval per application (variable interval)

[Alex Lutay]

Hi,

It is a wish-list request.

Data Platform develops a lot of charms for statfull DB applications.
For the list of reasons we have to change the default 'update-status-hook-interval', example:
> https://discourse.charmhub.io/t/mongodb-how-to-manage-units/8637

```
Further we highly suggest configuring the status hook to run frequently. In addition to reporting the primary, secondaries, and other statuses, the status hook performs self healing in the case of a network cut. To change the frequency of the update status hook do:

juju model-config update-status-hook-interval=<time(s/m/h)>

Note that this hook executes a read query to MongoDB. On a production level server this should be configured to occur at a frequency that doesn’t overload the server with read requests. Similarly the hook should not be configured at too quick of a frequency as this can delay other hooks from running. You can read more about status hooks here.
```

It is no way for production juju models with a lot of applications (e.g. Charmed Kubeflow). Most of those applications are stateless and do not require short `update-status` interval.

It is a request to create ability to make `update-status` configurable per application.

Another idea is to allow apps to request (temporary) shorter `update-status`, e.g. on handling backups (to show the progress), during cluster resilvering/reconfiguration, during installation, ...

The last proposal is to create socket/port/callback to report app/unit status from workload back to juju-agent (yeah, security, DDOS, ...).

When charm is idle, there is no need in short `update-status` interval, while during the workload reconfiguration activity the default interval 5 minutes is huge. It causes bugreports like:
https://github.com/canonical/mysql-router-k8s-operator/issues/12

Thank you in advance!

P.S. 'update-status-hook-interval' developed 6 years in #1697841:
https://github.com/juju/juju/pull/7519 (I was unable to find similar requests).