docs/configuration: Document external_labels option

Let the documentation generator understand the `labels.Labels` field, and
generate `config-file-reference.md` with

```console
$ go run tools/doc-generator \
> docs/configuration/config-file-reference.template > docs/configuration/config-file-reference.md
```

Signed-off-by: Benoît Knecht <[email protected]>

Author: BenoitKnecht

docs/configuration/config-file-reference.md

@@ -1181,6 +1181,9 @@ The `ruler_config` configures the Cortex ruler.
 # CLI flag: -ruler.external.url
 [external_url: <url> | default = ]
 
+# Labels to add to all alerts.
+[external_labels: <map of string to string> | default = ]
+
 ruler_client:
   # gRPC client max receive message size (bytes).
   # CLI flag: -ruler.client.grpc-max-recv-msg-size

tools/doc-generator/parser.go

@@ -257,6 +257,8 @@ func getFieldType(t reflect.Type) (string, error) {
 		return "string", nil
 	case "[]*relabel.Config":
 		return "relabel_config...", nil
+	case "labels.Labels":
+		return "map of string to string", nil
 	}
 
 	// Fallback to auto-detection of built-in data types