on config changed we are not creating the private key before generating the csr

[Abuelodelanada]

Enhancement Proposal

unit-grafana-0: 17:24:10.771 ERROR unit.grafana/0.juju-log certificates:31: Uncaught exception while in charm code:
Traceback (most recent call last):
  File "./src/charm.py", line 1480, in <module>
    main(GrafanaCharm, use_juju_for_storage=True)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/main.py", line 456, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/main.py", line 144, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 351, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 853, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 943, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-grafana-0/charm/lib/charms/tls_certificates_interface/v2/tls_certificates.py", line 1708, in _on_relation_broken
    self.on.all_certificates_invalidated.emit()
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 351, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 853, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-grafana-0/charm/venv/ops/framework.py", line 943, in _reemit
    custom_handler(event)
  File "/var/lib/juju/agents/unit-grafana-0/charm/lib/charms/tempo_k8s/v1/charm_tracing.py", line 538, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-grafana-0/charm/lib/charms/observability_libs/v1/cert_handler.py", line 399, in _on_all_certificates_invalidated
    self._generate_csr(overwrite=True, clear_cert=True)
  File "/var/lib/juju/agents/unit-grafana-0/charm/lib/charms/tempo_k8s/v1/charm_tracing.py", line 538, in wrapped_function
    return callable(*args, **kwargs)  # type: ignore
  File "/var/lib/juju/agents/unit-grafana-0/charm/lib/charms/observability_libs/v1/cert_handler.py", line 234, in _generate_csr
    raise RuntimeError(
RuntimeError: private key unset. call _generate_privkey() before you call this method.