(docs/internal-telemetry): add example for labels (open-telemetry#7257)

Co-authored-by: opentelemetrybot <[email protected]>
Co-authored-by: Tiffany Hrabusa <[email protected]>

Author: 3 people

content/en/docs/collector/internal-telemetry.md

@@ -71,6 +71,25 @@ service:
                 port: 8888
 ```
 
+If you want to add additional labels to the Prometheus metrics, you can add them
+with `prometheus::with_resource_constant_labels`:
+
+```yaml
+prometheus:
+  host: '0.0.0.0'
+  port: 8888
+  with_resource_constant_labels:
+    included:
+      - label_key
+```
+
+And then reference the labels in `service::telemetry::resource`:
+
+```yaml
+resource:
+  label_key: label_value
+```
+
 {{% alert title="Internal telemetry configuration changes" %}}
 
 As of Collector [v0.123.0], the `service::telemetry::metrics::address` setting