docs: add more information about enabling metrics

Signed-off-by: Bin Tang <[email protected]>

docs/configure_nydus.md

@@ -129,6 +129,8 @@ The Nydus snapshotter will get the new secret and parse the authorization. If yo
 ## Metrics
 
 Nydusd records metrics in its own format. The metrics are exported via a HTTP server on top of unix domain socket. Nydus-snapshotter fetches the metrics and convert them in to Prometheus format which is exported via a network address. Nydus-snapshotter by default does not fetch metrics from nydusd. You can enable the nydusd metrics download by assigning an network address to `metrics.address` in nydus-snapshotter's toml [configuration file](../misc/snapshotter/config.toml).
+Once this entry is enabled, not only nydusd metrics, but also some information about the nydus-snapshotter 
+runtime and snapshot related events are exported in Prometheus format as well.
 
 ## Diagnose