Reduce temp allocations in network metrics (#1895)

## Issue Addressed

Using `heaptrack` I could see that ~75% of Lighthouse temporary allocations are caused by temporary string allocations here.

## Proposed Changes

Reduces temporary `String` allocations when updating metrics in the `network` crate. The solution isn't perfect since we rebuild our caches with each call, but it's a significant improvement.

## Additional Info

NA

consensus/types/src/subnet_id.rs

@@ -75,3 +75,9 @@ impl Into<u64> for SubnetId {
         self.0
     }
 }
+
+impl Into<u64> for &SubnetId {
+    fn into(self) -> u64 {
+        self.0
+    }
+}