feat: Add per net device metrics

CHANGELOG.md

@@ -4,6 +4,12 @@
 
 ### Added
 
+- [#4145](https://github.com/firecracker-microvm/firecracker/pull/4145):
+  Added support for per net device metrics. In addition to aggregate metrics `net`,
+  each individual net device will emit metrics under the label `"net_{iface_id}"`.
+  E.g. the associated metrics for the endpoint `"/network-interfaces/eth0"` will
+  be available under `"net_eth0"` in the metrics json object.
+
 ### Changed
 
 - Simplified and clarified the removal policy of deprecated API elements

src/vmm/src/devices/mod.rs

@@ -17,14 +17,17 @@ pub mod virtio;
 pub use bus::{Bus, BusDevice, BusError};
 use log::error;
 
+use crate::devices::virtio::metrics::NetDeviceMetrics;
 use crate::devices::virtio::{QueueError, VsockError};
 use crate::logger::{IncMetric, METRICS};
 
 // Function used for reporting error in terms of logging
-// but also in terms of METRICS net event fails.
-pub(crate) fn report_net_event_fail(err: DeviceError) {
+// but also in terms of metrics of net event fails.
+// network metrics is reported per device so we need a handle to each net device's
+// metrics `net_iface_metrics` to report metrics for that device.
+pub(crate) fn report_net_event_fail(net_iface_metrics: &NetDeviceMetrics, err: DeviceError) {
     error!("{:?}", err);
-    METRICS.net.event_fails.inc();
+    net_iface_metrics.event_fails.inc();
 }
 
 pub(crate) fn report_balloon_event_fail(err: virtio::balloon::BalloonError) {