chore: simplify *Metrics::alloc and avoid .unwrap()

Reduce number of unwraps/lock operations needed by using the `.entry()`
API for more idiomatic code.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/vmm/src/devices/virtio/block/virtio/metrics.rs

@@ -100,20 +100,14 @@ impl BlockMetricsPerDevice {
     /// lock is always initialized so it is safe the unwrap
     /// the lock without a check.
     pub fn alloc(drive_id: String) -> Arc<BlockDeviceMetrics> {
-        if METRICS.read().unwrap().metrics.get(&drive_id).is_none() {
+        Arc::clone(
             METRICS
                 .write()
                 .unwrap()
                 .metrics
-                .insert(drive_id.clone(), Arc::new(BlockDeviceMetrics::new()));
-        }
-        METRICS
-            .read()
-            .unwrap()
-            .metrics
-            .get(&drive_id)
-            .unwrap()
-            .clone()
+                .entry(drive_id)
+                .or_insert_with(|| Arc::new(BlockDeviceMetrics::default())),
+        )
     }
 }
 

src/vmm/src/devices/virtio/net/metrics.rs

@@ -102,20 +102,14 @@ impl NetMetricsPerDevice {
     /// lock is always initialized so it is safe the unwrap
     /// the lock without a check.
     pub fn alloc(iface_id: String) -> Arc<NetDeviceMetrics> {
-        if METRICS.read().unwrap().metrics.get(&iface_id).is_none() {
+        Arc::clone(
             METRICS
                 .write()
                 .unwrap()
                 .metrics
-                .insert(iface_id.clone(), Arc::new(NetDeviceMetrics::new()));
-        }
-        METRICS
-            .read()
-            .unwrap()
-            .metrics
-            .get(&iface_id)
-            .unwrap()
-            .clone()
+                .entry(iface_id)
+                .or_insert_with(|| Arc::new(NetDeviceMetrics::default())),
+        )
     }
 }
 

src/vmm/src/devices/virtio/vhost_user_metrics.rs

@@ -95,19 +95,14 @@ impl VhostUserMetricsPerDevice {
     /// lock is always initialized so it is safe the unwrap
     /// the lock without a check.
     pub fn alloc(drive_id: String) -> Arc<VhostUserDeviceMetrics> {
-        if METRICS.read().unwrap().metrics.get(&drive_id).is_none() {
-            METRICS.write().unwrap().metrics.insert(
-                drive_id.clone(),
-                Arc::new(VhostUserDeviceMetrics::default()),
-            );
-        }
-        METRICS
-            .read()
-            .unwrap()
-            .metrics
-            .get(&drive_id)
-            .unwrap()
-            .clone()
+        Arc::clone(
+            METRICS
+                .write()
+                .unwrap()
+                .metrics
+                .entry(drive_id)
+                .or_insert_with(|| Arc::new(VhostUserDeviceMetrics::default())),
+        )
     }
 }