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/in_memory_backend.rs

@@ -20,6 +20,7 @@ use crate::{
 	StorageKey, StorageValue, StorageCollection,
 	trie_backend::TrieBackend,
 	backend::{Backend, insert_into_memory_db},
+	stats::UsageInfo,
 };
 use std::{error, fmt, collections::{BTreeMap, HashMap}, marker::PhantomData, ops};
 use hash_db::Hasher;
@@ -357,6 +358,12 @@ impl<H: Hasher> Backend<H> for InMemory<H> where H::Out: Codec {
 		self.trie = Some(TrieBackend::new(mdb, root));
 		self.trie.as_ref()
 	}
+
+	fn register_overlay_stats(&mut self, _stats: &crate::stats::StateMachineStats) { }
+
+	fn usage_info(&self) -> UsageInfo {
+		UsageInfo::empty()
+	}
 }
 
 #[cfg(test)]