Update monitoring documentation

doc/monitoring.md

@@ -28,3 +28,27 @@ $ openwec heartbeats -a 192.168.1.0 -s my-test-subscription
 ```
 
 Two formats are available: `text` (default) and `json` (`--format`).
+
+## Prometheus-compatible endpoint
+
+OpenWEC can expose a Prometheus-compatible endpoint with multiple metrics.
+
+### Configuration
+
+This feature is **disabled** by default.
+
+Metrics collection and publication can be enabled in the OpenWEC settings (see `monitoring` section of [openwec.conf.sample.toml](../openwec.conf.sample.toml)).
+
+### Available metrics
+
+| **Metric** | **Type** | **Labels** | **Description** |
+|---|---|---|---|
+| `openwec_received_events_total` | `Counter` | `subscription_uuid`, `subscription_name`, `machine` (optional*) | Number of events received by openwec |
+| `openwec_event_size_bytes_total` | `Counter` | `subscription_uuid`, `subscription_name`, `machine` (optional*) | The total size of all events received by openwec |
+| `http_request_body_real_size_bytes_total` | `Counter` | `method`, `uri`, `machine` (optional*) | The total size of all http requests body received by openwec after decryption and decompression |
+| `http_request_body_network_size_bytes_total` | `Counter` | `method`, `uri`, `machine` (optional*) | The total size of all http requests body received by openwec |
+| `openwec_messages_total` | `Counter` | `action` (one of `"enumerate"`, `"heartbeat"`, `"events"`) | Number of messages received by openwec |
+| `http_request_duration_seconds` | `Histogram` | `method`, `status`, `uri` | HTTP requests duration histogram |
+
+> [!WARNING]  
+> Enabling the `machine` labels may cause a **huge** increase in metric cardinality!