[charm_tracing] Add juju_charm to topology attributes

[mmkay]

Enhancement Proposal

In charm_tracing, we create resource attributes for the root span:

        resource = Resource.create(
            attributes={
(...)
                # juju topology
                "juju_unit": unit_name,
                "juju_application": self.app.name,
                "juju_model": self.model.name,
                "juju_model_uuid": self.model.uuid,
            }
        )

They don't contain the charm name itself. Should it be added?

[PietroPasotti]

I can't recall why I didn't add it. I think it didn't sound especially useful given we already have charm type and unit.
We could do that though.