Protect against load balancer service existing

In case something happens to cause our charm to go down without properly
cleaning up the service, use `client.apply()` instead of
client.create()`.

`apply()` will ensure the service configuration matches what is expected,
but is OK if it already exists, unlike `create()` which will throw an
error if the service already exists.

This will also prevent errors when scaling up (even though scaling isn't
supported at this time).

src/charm.py

@@ -59,6 +59,9 @@
 PFCP_PORT = 8805
 REQUIRED_CPU_EXTENSIONS = ["avx2", "rdrand"]
 
+# The default field manager set when using kubectl to create resources
+DEFAULT_FIELD_MANAGER = "controller"
+
 
 class IncompatibleCPUError(Exception):
     """Custom error to be raised when CPU doesn't support required instructions."""
@@ -168,8 +171,8 @@ def _create_external_upf_service(self) -> None:
             ),
         )
 
-        client.create(service)
-        logger.info("Created external UPF service")
+        client.apply(service, field_manager=DEFAULT_FIELD_MANAGER)
+        logger.info("Created/asserted existence of the external UPF service")
 
     def _on_remove(self, event: RemoveEvent) -> None:
         self._delete_external_upf_service()

tests/unit/test_charm.py

@@ -693,7 +693,9 @@ def test_when_install_then_external_service_is_created(self, patch_client):
             ),
         )
 
-        patch_client.return_value.create.assert_called_once_with(expected_service)
+        patch_client.return_value.apply.assert_called_once_with(
+            expected_service, field_manager="controller"
+        )
 
     @patch("charm.Client")
     def test_when_remove_then_external_service_is_deleted(self, patch_client):