changelog(metrics): Add vhost user device metrics

Report addition of vhost user device metrics with an example.

Signed-off-by: Sudan Landge <sudanl@amazon.com>

CHANGELOG.md

@@ -22,6 +22,12 @@
 - [#4063](https://github.com/firecracker-microvm/firecracker/pull/4063):
   Adds source-level instrumentation based tracing. See
   [tracing](./docs/tracing.md) for more details.
+- [#4226](https://github.com/firecracker-microvm/firecracker/pull/4226):
+  Added support for vhost user device metrics. Each vhost user device will
+  emit metrics under the label `"vhost_user_{device}_{drive_id}"`.
+  E.g. the associated metrics for vhost use block device with endpoint
+  `"/driv/rootfs"` will be available under `"vhost_user_block_rootfs"`
+  in the metrics json object.
 
 ### Changed