trie: add metric to track cache unloading (#3175)

trie/hasher.go

@@ -60,6 +60,7 @@ func (h *hasher) hash(n node, db DatabaseWriter, force bool) (node, node, error)
 		if n.canUnload(h.cachegen, h.cachelimit) {
 			// Unload the node from cache. All of its subnodes will have a lower or equal
 			// cache generation number.
+			cacheUnloadCounter.Inc(1)
 			return hash, hash, nil
 		}
 		if !dirty {