chore: apply suggestions from CR

src/metric-engine/src/engine.rs

@@ -163,9 +163,9 @@ impl RegionEngine for MetricEngine {
     /// Note: Returns `None` if it's a logical region.
     async fn region_disk_usage(&self, region_id: RegionId) -> Option<i64> {
         if self.inner.is_physical_region(region_id).await {
-            None
-        } else {
             self.inner.mito.region_disk_usage(region_id).await
+        } else {
+            None
         }
     }
 
@@ -203,9 +203,9 @@ impl RegionEngine for MetricEngine {
     /// Note: Returns `None` if it's a logical region.
     async fn role(&self, region_id: RegionId) -> Option<RegionRole> {
         if self.inner.is_physical_region(region_id).await {
-            None
-        } else {
             self.inner.mito.role(region_id).await
+        } else {
+            None
         }
     }
 
@@ -319,7 +319,7 @@ mod test {
         let physical_region_id = env.default_physical_region_id();
 
         assert!(env.metric().role(logical_region_id).await.is_none());
-        assert!(env.metric().role(physical_region_id).await.is_none());
+        assert!(env.metric().role(physical_region_id).await.is_some());
     }
 
     #[tokio::test]
@@ -339,6 +339,6 @@ mod test {
             .metric()
             .region_disk_usage(physical_region_id)
             .await
-            .is_none());
+            .is_some());
     }
 }