auto-generate prometheus.yml for workers metrics

docs/configuring-playbook-prometheus-grafana.md

@@ -62,6 +62,34 @@ Name | Description
 
 If you are using workers (`matrix_synapse_workers_enabled`) and have enabled `matrix_nginx_proxy_proxy_synapse_metrics` as described above, the playbook will also automatically proxy the all worker threads's metrics to `https://matrix.DOMAIN/_synapse-worker-TYPE-ID/metrics`, where `TYPE` corresponds to the type and `ID` to the instanceId of a worker as exemplified in `matrix_synapse_workers_enabled_list`.
 
+The playbook also generates an exemplary prometheus.yml config file (`matrix_base_data_path/external_prometheus.yml.template`) with all the correct paths which you can copy to your Prometheus server and adapt to your needs, especially edit the specified `password_file` path and contents and path to your `synapse-v2.rules`.
+It will look a bit like this:
+```yaml
+scrape_configs:
+  - job_name: 'synapse'
+    metrics_path: /_synapse/metrics
+    scheme: https
+    basic_auth:
+      username: prometheus
+      password_file: /etc/prometheus/password.pwd
+    static_configs:
+      - targets: ['matrix.DOMAIN:443']
+        labels:
+          job: "master"
+          index: 1
+  - job_name: 'synapse-generic_worker-1'
+    metrics_path: /_synapse-worker-generic_worker-18111/metrics
+    scheme: https
+    basic_auth:
+      username: prometheus
+      password_file: /etc/prometheus/password.pwd
+    static_configs:
+      - targets: ['matrix.DOMAIN:443']
+        labels:
+          job: "generic_worker"
+          index: 18111
+```
+
 ### Collecting system and Postgres metrics to an external Prometheus server (advanced)
 
 When you normally enable the Prometheus and Grafana via the playbook, it will also show general system (via node-exporter) and Postgres (via postgres-exporter) stats. If you are instead collecting your metrics to an external Prometheus server, you can follow this advanced configuration example to also export these stats.