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/README.md

@@ -43,5 +43,7 @@ If monitoring is enabled (via the *prometheus* directive) then the following met
 
 * coredns_dnssec_cache_size{type} - total elements in the cache, type is "signature".
 * coredns_dnssec_cache_capacity{type} - total capacity of the cache, type is "signature".
+* coredns_dnssec_cache_hits_total - Counter of cache hits.
+* coredns_dnssec_cache_misses_total - Counter of cache misses.
 
 ## Examples

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
 }
 

middleware/dnssec/handler.go

@@ -53,6 +53,20 @@ var (
 		Name:      "cache_capacity",
 		Help:      "The dnssec cache's capacity.",
 	}, []string{"type"})
+
+	cacheHits = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "cache_hits_total",
+		Help:      "The count of cache hits.",
+	})
+
+	cacheMisses = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "cache_misses_total",
+		Help:      "The count of cache misses.",
+	})
 )
 
 // Name implements the Handler interface.
@@ -63,4 +77,6 @@ const subsystem = "dnssec"
 func init() {
 	prometheus.MustRegister(cacheSize)
 	prometheus.MustRegister(cacheCapacity)
+	prometheus.MustRegister(cacheHits)
+	prometheus.MustRegister(cacheMisses)
 }