cannot sequentially create multiple cross model relations from the same app

[Pietro Pasotti]

when you have multiple cross-model relations you need to first create all offers (and consume them), and only then create the relations with `juju relate`.

Creating an offer, relating it, and then creating another offer gives an error such as:
ERROR cannot update application offer "gagent": application endpoint "logging-consumer" has active consumers

Remark from Joe Phillips:
> The bug comes from [here](https://github.com/juju/juju/blob/fc069f19f894c4f7c777ed9a290cd64a706c24dd/state/applicationoffers.go#L679). I don't think the determination by set difference is correct.

Steps to reproduce:

deploy cos-lite in a kubernetes model

deploy in a different (machine?) controller any charm supporting a cos-agent subordinate such as https://github.com/canonical/intro-to-charming-mc/tree/part-3-observability

deploy grafana-agent and attach it to the charm
follow the steps in https://discourse.charmhub.io/t/using-the-grafana-agent-machine-charm/8896#step-4-relate-grafana-agent-to-cos-lite-prometheus-loki-and-grafana to create one of the CMRs and then another

--> bug.

Reproducible in a setup with:

juju snap version 3.1.7
juju machine model agent version 3.2.3
juju kubernetes model agent version 3.1.6