middleware/cache: Add metrics (#132)

Add prometheus metrics to the cache handler. This just used prometheus,
if the metrics middleware does not setup the handler, there is nobody
reading these metrics, but they are still reported. Seems the simplest
solution while keeping the whole middleware separation in tact.

middleware/cache/handler.go

@@ -4,6 +4,7 @@ import (
 	"github.com/miekg/coredns/middleware"
 
 	"github.com/miekg/dns"
+	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/net/context"
 )
 
@@ -24,8 +25,12 @@ func (c Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (
 		resp := i.toMsg(r)
 		state.SizeAndDo(resp)
 		w.WriteMsg(resp)
+
+		cacheHitCount.WithLabelValues(zone).Inc()
 		return dns.RcodeSuccess, nil
 	}
+	cacheMissCount.WithLabelValues(zone).Inc()
+
 	crr := NewCachingResponseWriter(w, c.cache, c.cap)
 	return c.Next.ServeDNS(ctx, crr, r)
 }
@@ -42,3 +47,26 @@ func (c Cache) Get(qname string, qtype uint16, do bool) (*item, bool) {
 	}
 	return nil, false
 }
+
+var (
+	cacheHitCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "hit_count_total",
+		Help:      "Counter of DNS requests that were found in the cache.",
+	}, []string{"zone"})
+
+	cacheMissCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "miss_count_total",
+		Help:      "Counter of DNS requests that were not found in the cache.",
+	}, []string{"zone"})
+)
+
+const subsystem = "cache"
+
+func init() {
+	prometheus.MustRegister(cacheHitCount)
+	prometheus.MustRegister(cacheMissCount)
+}