prometheus: Fix scraping prometheus’ own metrics

prometheus/defaults/main.yml

 ---
 prometheus_args: ""
+prometheus_metrics_path: /metrics
 # This variable intentionally left null.  They need to exist, so that the
 # template works, but they need to have a value (empty string does not work).
 prometheus_rules: null

prometheus/templates/prometheus.yml.j2

@@ -18,6 +18,7 @@ scrape_configs:
     # Override the global default and scrape targets from this job every 5 seconds.
     scrape_interval: 5s
     scrape_timeout: 5s
+    metrics_path: '{{ prometheus_metrics_path }}'
     static_configs:
       - targets:
         - 'localhost:9090'