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

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 }}"