Add metrics for inmemory canonical state

[mattsse]

we should have some metrics for this that tracks

• number of entries

reth/crates/chain-state/src/in_memory.rs

Lines 24 to 32 in c1b5410

	#[derive(Debug, Default)]
	pub(crate) struct InMemoryState {
	/// All canonical blocks that are not on disk yet.
	blocks: RwLock<HashMap<B256, Arc<BlockState>>>,
	/// Mapping of block numbers to block hashes.
	numbers: RwLock<HashMap<u64, B256>>,
	/// The pending block that has not yet been made canonical.
	pending: RwLock<Option<BlockState>>,
	}