Added step to upgrade unit by running openstack-upgrade action.

cou/apps/base.py

@@ -665,6 +665,21 @@ def _get_resume_unit_step(self, unit: ApplicationUnit) -> UnitUpgradeStep:
             ),
         )
 
+    def _get_openstack_upgrade_step(self, unit: ApplicationUnit) -> UnitUpgradeStep:
+        """Get the step to upgrade a unit.
+
+        :param unit: Unit to be upgraded.
+        :type unit: ApplicationUnit
+        :return: Step to upgrade a unit.
+        :rtype: UnitUpgradeStep
+        """
+        return UnitUpgradeStep(
+            description=(f"Upgrade the unit: '{unit.name}'."),
+            coro=self.model.run_action(
+                unit_name=unit.name, action_name="openstack-upgrade", raise_on_failure=True
+            ),
+        )
+
     def _get_workload_upgrade_step(self, target: OpenStackRelease) -> UpgradeStep:
         """Get workload upgrade step by changing openstack-origin or source.
 

tests/unit/apps/test_base.py

@@ -100,3 +100,22 @@ def test_get_resume_unit_step(model):
 
     app = OpenStackApplication(app_name, status, {}, model, charm, {})
     assert app._get_resume_unit_step(unit) == expected_upgrade_step
+
+
+def test_get_openstack_upgrade_step(model):
+    charm = "app"
+    app_name = "my_app"
+    status = MagicMock(spec_set=ApplicationStatus())
+    status.charm_channel = "ussuri/stable"
+
+    unit = ApplicationUnit("my_app/0", MagicMock(), MagicMock(), MagicMock())
+
+    expected_upgrade_step = UnitUpgradeStep(
+        description=f"Upgrade the unit: '{unit.name}'.",
+        coro=model.run_action(
+            unit_name=unit.name, action_name="openstack-upgrade", raise_on_failure=True
+        ),
+    )
+
+    app = OpenStackApplication(app_name, status, {}, model, charm, {})
+    assert app._get_openstack_upgrade_step(unit) == expected_upgrade_step