s3-integrator should also observe `relation-created` events

[phvalguima]

The following setup:

juju deploy app1
juju deploy app2
juju deploy s3-integrator --config .... # some config

# And relating
juju integrate app1 s3-integrator
juju integrate app2 s3-integrator

Should result in both app1 and app2 receiving the same data. However, what happens is that only the first really related app gets the data whereas the second gets an empty "data: {}" field.

Looking into the code, I think we are not watching for new apps being attached (i.e. the -created events). We should essentially call the _on_config_changed logic, but focusing only on the new app at every -created.

[github-actions[bot]]

https://warthogs.atlassian.net/browse/DPE-4212

[taurus-forever]

@phvalguima can you please post here the summary of our long talk in MM. Just for the history. Tnx!

[phvalguima]

To summarize our internal discussions here, most of the use cases do not need s3-integrator with more than one relation. That also contemplates the cases where we have multi-cluster deployments:

• single juju app, single sharded app - single s3-integrator
• multi juju apps, async replications (hence, multiple clusters) - different s3-integrators, one per cluster
• multi juju apps, sharded app (hence, compose one single big cluster) - (temporary) single s3-integrator related to the main orchestrator of that bigger cluster

Now, there is one use-case raised by @deusebio which is not yet clear if it establishes the need for single s3-integrator / multiple relations:

use-case where a s3 bucket is shared by multiple apps. Spark write logs to an s3 buckets that needs to be read by spark history server. Therefore a s3 integrator (provider) is related to both spark-history-server (requirer) and configuration hub (requirer)

In case @deusebio's case is kept and we will use a single s3-integrator charm for that, then we need to:

1. remove the limit from metadata.yaml, if it was added by then
2. Review and merge #33

If this is not the case, then this bug can be closed.