prometheus: add matrix metrics and dashboard

The dashboards is from [1] and fixed with:
sed 's/${DS_PROMETHEUS}/$datasource/g' -i synapse.json

[1] c167e09fe5/contrib/grafana

Closes: #290

Signed-off-by: Leonidas Spyropoulos <artafinde@gmail.com>

roles/prometheus/defaults/main.yml

@@ -60,3 +60,9 @@ blackbox_targets:
   smtp_starttls:
     - mail.archlinux.org:25
     - lists.archlinux.org:25
+matrix_metrics_endpoints:
+  - homeserver
+  - appservice
+  - federation_reader
+  - federation_writer
+  - media_repository

roles/prometheus/templates/prometheus.yml.j2

@@ -153,4 +153,16 @@ scrape_configs:
       - target_label: __address__
         replacement: 127.0.0.1:9115
 {% endfor %}
+{% for endpoint in matrix_metrics_endpoints %}
+
+  - job_name: 'synapse_{{ endpoint }}'
+    scrape_interval: 15s
+    metrics_path: "/metrics/synapse.{{ endpoint }}"
+    scheme: https
+    basic_auth:
+      username: {{ vault_matrix_secrets.metrics_user }}
+      password: {{ vault_matrix_secrets.metrics_password }}
+    static_configs:
+      - targets: ["matrix.archlinux.org:443"]
+{% endfor %}
 {% endif %}