Document Prometheus

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+# Prometheus Metrics
+
+Pleroma includes support for exporting metrics via the [prometheus_ex](https://github.com/deadtrickster/prometheus.ex) library.
+
+## `/api/pleroma/app_metrics`
+### Exports Prometheus application metrics
+* Method: `GET`
+* Authentication: not required
+* Params: none
+* Response: JSON
+
+## Grafana
+### Config example
+The following is a config example to use with [Grafana](https://grafana.com)
+
+```
+  - job_name: 'beam'
+    metrics_path: /api/pleroma/app_metrics
+    scheme: https
+    static_configs:
+    - targets: ['pleroma.soykaf.com']
+```