Fix PERSIST_ETH1_CACHE / PERSIST_OP_POOL Metrics (#4278)

Do these metrics ever get read? As far as I'm aware, they're only ever updated when lighthouse is shutting down?

Author: ethDreamer

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -555,7 +555,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
     /// Persists `self.eth1_chain` and its caches to disk.
     pub fn persist_eth1_cache(&self) -> Result<(), Error> {
-        let _timer = metrics::start_timer(&metrics::PERSIST_OP_POOL);
+        let _timer = metrics::start_timer(&metrics::PERSIST_ETH1_CACHE);
 
         if let Some(eth1_chain) = self.eth1_chain.as_ref() {
             self.store