Make --tags=setup-prometheus not break, relying on matrix-base facts

roles/matrix-base/tasks/setup_matrix_base.yml

@@ -23,7 +23,6 @@
   docker_network:
     name: "{{ matrix_docker_network }}"
     driver: bridge
-  register: matrix_docker_network_info
 
 - name: Ensure matrix-remove-all script created
   template:

roles/matrix-prometheus/defaults/main.yml

@@ -26,6 +26,9 @@ matrix_prometheus_systemd_wanted_services_list: []
 # Takes an "<ip>:<port>" or "<port>" value (e.g. "127.0.0.1:9090"), or empty string to not expose.
 matrix_prometheus_container_http_host_bind_port: ''
 
+# Target addresses for the "node" scraper configuration.
+# Unless you define this as a non-empty list, it gets populated at runtime with the IP address of `matrix-prometheus-node-exporter` and port 9100.
+matrix_prometheus_endpoint_node_targets: []
 
 # Default prometheus configuration template which covers the generic use case.
 # You can customize it by controlling the various variables inside it.

roles/matrix-prometheus/tasks/setup.yml

@@ -24,7 +24,22 @@
     - "{{ matrix_prometheus_config_path }}"
     - "{{ matrix_prometheus_data_path }}"
   when: matrix_prometheus_enabled|bool
-  
+
+- block:
+    # Well, this actually creates the network if it doesn't exist, but..
+    # The network should have been created by `matrix-base` already.
+    # We don't rely on that other call and its result, because it runs
+    # on `--tags=setup-all`, but will get skipped during `--tags=setup-prometheus`.
+    - name: Fetch Matrix Docker network details
+      docker_network:
+        name: "{{ matrix_docker_network }}"
+        driver: bridge
+      register: matrix_docker_network_info
+
+    - set_fact:
+        matrix_prometheus_endpoint_node_targets: ["{{ matrix_docker_network_info.network.IPAM.Config[0].Gateway }}:9100"]
+  when: "matrix_prometheus_enabled|bool and matrix_prometheus_node_exporter_enabled|bool and matrix_prometheus_endpoint_node_targets|length == 0"
+
 - name: Ensure prometheus.yml installed
   copy:
     content: "{{ matrix_prometheus_configuration|to_nice_yaml }}"

roles/matrix-prometheus/templates/prometheus.yml.j2

@@ -36,5 +36,5 @@ scrape_configs:
   {% if matrix_prometheus_node_exporter_enabled %}
   - job_name: node
     static_configs:
-      - targets: ['{{ matrix_docker_network_info.network.IPAM.Config[0].Gateway }}:9100']
+      - targets: {{ matrix_prometheus_endpoint_node_targets|to_json }}
   {% endif %}