DataDog and StatsD Metrics Support

* Added support for DataDog and StatsD monitoring
* Added documentation

middlewares/datadog_test.go

@@ -0,0 +1,53 @@
+package middlewares
+
+import (
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+
+	"github.com/containous/traefik/testhelpers"
+	"github.com/containous/traefik/types"
+	"github.com/stvp/go-udp-testing"
+	"github.com/urfave/negroni"
+)
+
+func TestDatadog(t *testing.T) {
+	udp.SetAddr(":18125")
+	// This is needed to make sure that UDP Listener listens for data a bit longer, otherwise it will quit after a millisecond
+	udp.Timeout = 5 * time.Second
+	recorder := httptest.NewRecorder()
+	InitDatadogClient(&types.Datadog{":18125", "1s"})
+
+	n := negroni.New()
+	dd := NewDataDog("test")
+	defer StopDatadogClient()
+	metricsMiddlewareBackend := NewMetricsWrapper(dd)
+
+	n.Use(metricsMiddlewareBackend)
+	r := http.NewServeMux()
+	r.HandleFunc(`/ok`, func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		fmt.Fprintln(w, "ok")
+	})
+	r.HandleFunc(`/not-found`, func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+		fmt.Fprintln(w, "not-found")
+	})
+	n.UseHandler(r)
+
+	req1 := testhelpers.MustNewRequest(http.MethodGet, "http://localhost:3000/ok", nil)
+	req2 := testhelpers.MustNewRequest(http.MethodGet, "http://localhost:3000/not-found", nil)
+
+	expected := []string{
+		// We are only validating counts, as it is nearly impossible to validate latency, since it varies every run
+		"traefik.requests.total:1.000000|c|#service:test,code:404,method:GET\n",
+		"traefik.requests.total:1.000000|c|#service:test,code:200,method:GET\n",
+	}
+
+	udp.ShouldReceiveAll(t, expected, func() {
+		n.ServeHTTP(recorder, req1)
+		n.ServeHTTP(recorder, req2)
+	})
+}