refactor: remove multiple relation check in charm code.

[chanchiwai-ray]

The metadata.yaml support limit for interface, this can limits how many relation the interface can be connected to, and we can handle the relation checks in juju level.

Also removed the incorrect usage of envdir in tox.ini this is no longer working in tox >= 4.

Old behavior:

$ juju status
hardware-observer/1*  active    blocked            10.42.75.15            Cannot relate to more than one grafana-agent

New behavior:

$ juju relate g2  hardware-observer
ERROR cannot add relation "g2:cos-agent hardware-observer:cos-agent": establishing a new relation for hardware-observer:cos-agent would exceed its maximum relation limit of 1 (quota limit exceeded)

Closes: #144

[dashmage]

Although I'm in favor of the changes, I think it would be nice to remove the get_num_cos_agent_relations method as well since there would only be either 1 or 0 cos-agent relations now (ie, the relation is either present/ absent). It might be better to have the method be cos_agent_relation_present and use something like self.cos_agent_relation_present == True instead of handling the number of relations now. But if you feel like it expands the scope of the PR too much, we can handle it on separate PR as well 😄

[chanchiwai-ray]

Although I'm in favor of the changes, I think it would be nice to remove the get_num_cos_agent_relations method as well since there would only be either 1 or 0 cos-agent relations now (ie, the relation is either present/ absent). It might be better to have the method be cos_agent_relation_present and use something like self.cos_agent_relation_present == True instead of handling the number of relations now. But if you feel like it expands the scope of the PR too much, we can handle it on separate PR as well 😄

good point, the property can be removed and replace with the function call directly. (save few lines of code)

[chanchiwai-ray]

Rebased onto master branch

[dashmage]

Func tests are failing due to this issue: charmed-kubernetes/pytest-operator#131

Edit: I've provided a temporary fix with this PR: #232
Edit2: Fix not needed anymore. The func tests work with pytest-asyncio==0.21.2

The base branch was changed.

[chanchiwai-ray]

rebased onto main