Add reminder to remove custom blackbox endpoints

doc/source/operations/upgrading-openstack.rst

@@ -106,6 +106,39 @@ the following in ``kayobe-config/etc/kayobe/stackhpc-monitoring.yml``:
    # targets being templated during deployment.
    stackhpc_enable_os_capacity: false
 
+Prometheus blackbox exporter endpoints
+--------------------------------------
+
+Many endpoints for the Blackbox exporter are now templated in the Kolla-Ansible
+group vars for the cloud. This means that the
+``prometheus_blackbox_exporter_endpoints`` variable can be removed from the
+environment's ``kolla/globals.yml`` file (if applicable) and the endpoints will
+fallback to the ones templated in the group vars. Backend endpoints such as
+`these <https://github.com/stackhpc/stackhpc-kayobe-config/blob/094c2e012a037309d103c08a71eb633fdeb214e7/etc/kayobe/kolla/inventory/group_vars/prometheus-blackbox-exporter#L27-L64>`__
+are not yet templated by Kolla-Ansible.
+
+Additional endpoints may still be added.
+
+For Kolla-Ansible templating, use ``stackhpc_prometheus_blackbox_exporter_endpoints_custom``.
+For example:
+
+.. code-block:: yaml
+   :caption: ``etc/kayobe/kolla/inventory/group_vars/prometheus-blackbox-exporter``
+
+   stackhpc_prometheus_blackbox_exporter_endpoints_custom:
+     - 'custom_service:http_2xx:{{ public_protocol }}://{{ external_fqdn | put_address_in_context('url') }}:{{ custom_serivce_port }}'
+
+Alternatively, for Kayobe templating, use the ``prometheus_blackbox_exporter_endpoints_kayobe`` variable.
+For example:
+
+.. code-block:: yaml
+   :caption: ``kolla/globals.yml``
+
+   prometheus_blackbox_exporter_endpoints_kayobe:
+      - endpoints:
+         - "pulp:http_2xx:{{ pulp_url }}/pulp/api/v3/status/"
+      enabled: "{{ seed_pulp_container_enabled | bool }}"
+
 Known issues
 ============