Prometheus support (#450)

Co-authored-by: nils måsén <nils@piksel.se>
Co-authored-by: MihailITPlace <ya.halo-halo@yandex.ru>
Co-authored-by: Sebastiaan Tammer <sebastiaantammer@gmail.com>

docs/arguments.md

@@ -164,7 +164,7 @@ Environment Variable: WATCHTOWER_LABEL_ENABLE
 ## Without updating containers
 Will only monitor for new images, send notifications and invoke the [pre-check/post-check hooks](https://containrrr.dev/watchtower/lifecycle-hooks/), but will **not** update the containers.
 
-> ### ⚠️ Please note
+> **⚠️ Please note**
 >
 > Due to Docker API limitations the latest image will still be pulled from the registry.
 
@@ -238,9 +238,7 @@ Sets an authentication token to HTTP API requests.
 Environment Variable: WATCHTOWER_HTTP_API_TOKEN
                 Type: String
              Default: -
-```
-
-## Filter by scope
+```## Filter by scope
 Update containers that have a `com.centurylinklabs.watchtower.scope` label set with the same value as the given argument. This enables [running multiple instances](https://containrrr.github.io/watchtower/running-multiple-instances).
 
 ```
@@ -250,6 +248,16 @@ Environment Variable: WATCHTOWER_SCOPE
              Default: -
 ``` 
 
+## HTTP API Metrics
+Enables a metrics endpoint, exposing prometheus metrics via HTTP. See [Metrics](metrics.md) for details.  
+
+```
+            Argument: --http-api-metrics
+Environment Variable: WATCHTOWER_HTTP_API_METRICS
+                Type: Boolean
+             Default: false
+```
+
 ## Scheduling
 [Cron expression](https://pkg.go.dev/github.com/robfig/cron@v1.2.0?tab=doc#hdr-CRON_Expression_Format) in 6 fields (rather than the traditional 5) which defines when and how often to check for new images. Either `--interval` or the schedule expression
 can be defined, but not both. An example: `--schedule "0 0 4 * * *"`