feat(monitoring): add grafana, prometheus, node_exporter roles

- grafana: postgres backend, provisioning (prometheus datasource + node/NUT dashboards)
- prometheus: native service, scrape config via host_vars, localhost bind
- node_exporter: host metrics exporter (localhost:9100)

roles/prometheus/templates/nginx-vhost.conf.j2

@@ -0,0 +1,30 @@
+server {
+    listen 80;
+    listen [::]:80;
+    server_name {{ prometheus_nginx_server_name }};
+    return 301 https://$server_name$request_uri;
+}
+
+server {
+    listen 443 ssl http2;
+    listen [::]:443 ssl http2;
+    server_name {{ prometheus_nginx_server_name }};
+
+    ssl_certificate {{ prometheus_nginx_ssl_certificate }};
+    ssl_certificate_key {{ prometheus_nginx_ssl_certificate_key }};
+
+    ssl_protocols TLSv1.2 TLSv1.3;
+    ssl_ciphers HIGH:!aNULL:!MD5;
+    ssl_prefer_server_ciphers on;
+
+    access_log /var/log/nginx/prometheus_access.log;
+    error_log /var/log/nginx/prometheus_error.log;
+
+    location / {
+        proxy_pass http://{{ prometheus_web_listen_address }};
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+}