Measure and expose DNS programming latency from Kubernetes plugin. (#3171)

For now metric is measure only for headless services. Informer has been slighlty
refactored, so the code can measure latency without storing extra fields on
Endpoint struct.

Signed-off-by: Janek Łukaszewicz <janluk@google.com>

Suggestions from code review

Co-Authored-By: Chris O'Haver <cohaver@infoblox.com>

plugin/kubernetes/setup.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 	"github.com/coredns/coredns/plugin/pkg/dnsutil"
 	clog "github.com/coredns/coredns/plugin/pkg/log"
 	"github.com/coredns/coredns/plugin/pkg/parse"
@@ -51,6 +52,11 @@ func setup(c *caddy.Controller) error {
 
 	k.RegisterKubeCache(c)
 
+	c.OnStartup(func() error {
+		metrics.MustRegister(c, DnsProgrammingLatency)
+		return nil
+	})
+
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		k.Next = next
 		return k