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 {

trie/trie.go

@@ -35,7 +35,10 @@ var (
 	emptyState common.Hash
 )
 
-var cacheMissCounter = metrics.NewRegisteredCounter("trie/cachemiss", nil)
+var (
+	cacheMissCounter   = metrics.NewRegisteredCounter("trie/cachemiss", nil)
+	cacheUnloadCounter = metrics.NewRegisteredCounter("trie/cacheunload", nil)
+)
 
 // CacheMisses retrieves a global counter measuring the number of cache misses
 // the trie did since process startup. This isn't useful for anything apart from