Document additional Prometheus settings (librenms#18544)

Added additional settings for Prometheus configuration.

Author: electrocret

doc/Extensions/metrics/Prometheus.md

@@ -43,6 +43,14 @@ If your pushgateway uses basic authentication, configure the following:
     lnms config:set prometheus.password password
     ```
 
+Additional settings
+
+!!! setting "poller/prometheus"
+    ```bash
+    lnms config:set prometheus.attach_sysname true
+    ```
+
+
 ## Prefix
 
 Setting the 'prefix' option will cause all metric names to begin with