State metrics possible changes (#5168)

* Registering state from overlay.

* fix

* fix2

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/primitives/state-machine/src/proving_backend.rs

@@ -283,6 +283,12 @@ impl<'a, S, H> Backend<H> for ProvingBackend<'a, S, H>
 	{
 		self.0.child_storage_root(storage_key, child_info, delta)
 	}
+
+	fn register_overlay_stats(&mut self, _stats: &crate::stats::StateMachineStats) { }
+
+	fn usage_info(&self) -> crate::stats::UsageInfo {
+		self.0.usage_info()
+	}
 }
 
 /// Create proof check backend.