document prom metrics exposed

README.md

@@ -74,6 +74,24 @@ TAILSCALESD_API_KEY=my-api-key
 TAILSCALESD_HOST=127.0.0.1
 ```
 
+### Monitoring
+
+
+This service provides the following Prometheus metrics:
+
+- `tailscalesd_unhandled_background_task_crashes`
+  - **Type**: Counter
+  - **Description**: The number of unhandled background task crashes. This counter increases every time a background task crashes without being handled by the application.
+- `tailscalesd_matrix_sd_down`
+  - **Type**: Counter
+  - **Description**: The number times a matrix sd host was unreachable. This counter increments each time a connection attempt to a matrix sd host fails.
+  - **Labels**:
+    - `device_hostname`: The hostname of the device that was unreachable.
+
+It also provides HTTP server metrics from [trallnag/prometheus-fastapi-instrumentator](https://github.com/trallnag/prometheus-fastapi-instrumentator)
+
+## Dev
+
 ### Running the Application
 
 1. **Install Dependencies**: Ensure that all required dependencies are installed.