add metrics in the example

Add metrics as well (good example to have).

README.md

@@ -15,6 +15,12 @@ writing CoreDNS plugins.
 example
 ~~~
 
+## Metrics
+
+If monitoring is enabled (via the *prometheus* directive) the following metric is exported:
+
+* `coredns_example_request_count_total{server}` - query count to the *example* plugin.
+
 ## Examples
 
 In this configuration, we forward all queries to 9.9.9.9 and print "example" whenever we recieve
@@ -26,3 +32,7 @@ a query.
   example
 }
 ```
+
+## Also See
+
+See the [manual](https://coredns.io/manual).

example.go

@@ -1,4 +1,6 @@
-// The example plugin prints example to stdout on every packet received.
+// Package example is a CoreDNS plugin that prints "example" to stdout on every packet received.
+//
+// It serves as an example CoreDNS plugin with numerous code comments.
 package example
 
 import (
@@ -7,6 +9,7 @@ import (
 	"os"
 
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 
 	"github.com/miekg/dns"
 	"golang.org/x/net/context"
@@ -28,6 +31,9 @@ func (e Example) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	// Wrap.
 	pw := NewResponsePrinter(w)
 
+	// Export metric with the server label set to the current server handling the request.
+	requestCount.WithLabelValues(metrics.WithServer(ctx)).Add(1)
+
 	// Call next plugin (if any).
 	return plugin.NextOrFailure(e.Name(), e.Next, ctx, pw, r)
 }

metrics.go

@@ -0,0 +1,20 @@
+package example
+
+import (
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// requestCount exports a prometheus metric that is incremented every time
+// a query is seen by the example plugin.
+var requestCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+	Namespace: plugin.Namespace,
+	Subsystem: "example",
+	Name:      "request_count_total",
+	Help:      "Counter of requests made.",
+}, []string{"server"})
+
+var once sync.Once

setup.go

@@ -3,6 +3,7 @@ package example
 import (
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics"
 
 	"github.com/mholt/caddy"
 )
@@ -27,6 +28,22 @@ func setup(c *caddy.Controller) error {
 		return plugin.Error("example", c.ArgErr())
 	}
 
+	// Add a startup function that will -- after all plugins have been loaded -- check if the
+	// prometheus plugin has been used - if so we will export metrics. We can only register
+	// this metric once, hence the "once.Do".
+	c.OnStartup(func() error {
+		once.Do(func() {
+			m := dnsserver.GetConfig(c).Handler("prometheus")
+			if m == nil {
+				return
+			}
+			if x, ok := m.(*metrics.Metrics); ok {
+				x.MustRegister(requestCount)
+			}
+		})
+		return nil
+	})
+
 	// Add the Plugin to CoreDNS, so Servers can use it in their plugin chain.
 	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
 		return Example{Next: next}