Add metrics for cache hits/misses (#375)

* Add metrics for cache hits/misses

Add counters for cache middleware hits and misses.

* Add test for cache middleware hit/miss counters.

* Fix cache hit metric incrementing.

* Add cache hit/miss metrics to dnssec middleware.

* Update README metric documentation.

middleware/dnssec/dnssec.go

@@ -115,8 +115,10 @@ func (d Dnssec) set(key string, sigs []dns.RR) {
 
 func (d Dnssec) get(key string) ([]dns.RR, bool) {
 	if s, ok := d.cache.Get(key); ok {
+		cacheHits.Inc()
 		return s.([]dns.RR), true
 	}
+	cacheMisses.Inc()
 	return nil, false
 }