Merge branch '1668-prometheus-access-restrictions' into 'develop'

[#1668] App metrics endpoint (Prometheus) access restrictions

Closes #1668

See merge request pleroma/pleroma!3093

docs/API/prometheus.md

@@ -2,15 +2,37 @@
 
 Pleroma includes support for exporting metrics via the [prometheus_ex](https://github.com/deadtrickster/prometheus.ex) library.
 
+Config example:
+
+```
+config :prometheus, Pleroma.Web.Endpoint.MetricsExporter,
+  enabled: true,
+  auth: {:basic, "myusername", "mypassword"},
+  ip_whitelist: ["127.0.0.1"],
+  path: "/api/pleroma/app_metrics",
+  format: :text
+```
+
+* `enabled` (Pleroma extension) enables the endpoint
+* `ip_whitelist` (Pleroma extension) could be used to restrict access only to specified IPs
+* `auth` sets the authentication (`false` for no auth; configurable to HTTP Basic Auth, see [prometheus-plugs](https://github.com/deadtrickster/prometheus-plugs#exporting) documentation)
+* `format` sets the output format (`:text` or `:protobuf`)
+* `path` sets the path to app metrics page 
+
+
 ## `/api/pleroma/app_metrics`
+
 ### Exports Prometheus application metrics
+
 * Method: `GET`
-* Authentication: not required
+* Authentication: not required by default (see configuration options above)
 * Params: none
-* Response: JSON
+* Response: text
 
 ## Grafana
+
 ### Config example
+
 The following is a config example to use with [Grafana](https://grafana.com)
 
 ```