Remove the word middleware (#1067)

* Rename middleware to plugin

first pass; mostly used 'sed', few spots where I manually changed
text.

This still builds a coredns binary.

* fmt error

* Rename AddMiddleware to AddPlugin

* Readd AddMiddleware to remain backwards compat

plugin/metrics/metrics_test.go

@@ -0,0 +1,83 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin"
+	mtest "github.com/coredns/coredns/plugin/metrics/test"
+	"github.com/coredns/coredns/plugin/pkg/dnsrecorder"
+	"github.com/coredns/coredns/plugin/test"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func TestMetrics(t *testing.T) {
+	met := &Metrics{Addr: "localhost:0", zoneMap: make(map[string]bool)}
+	if err := met.OnStartup(); err != nil {
+		t.Fatalf("Failed to start metrics handler: %s", err)
+	}
+	defer met.OnShutdown()
+
+	met.AddZone("example.org.")
+
+	tests := []struct {
+		next          plugin.Handler
+		qname         string
+		qtype         uint16
+		metric        string
+		expectedValue string
+	}{
+		// This all works because 1 bucket (1 zone, 1 type)
+		{
+			next:          test.NextHandler(dns.RcodeSuccess, nil),
+			qname:         "example.org",
+			metric:        "coredns_dns_request_count_total",
+			expectedValue: "1",
+		},
+		{
+			next:          test.NextHandler(dns.RcodeSuccess, nil),
+			qname:         "example.org",
+			metric:        "coredns_dns_request_count_total",
+			expectedValue: "2",
+		},
+		{
+			next:          test.NextHandler(dns.RcodeSuccess, nil),
+			qname:         "example.org",
+			metric:        "coredns_dns_request_type_count_total",
+			expectedValue: "3",
+		},
+		{
+			next:          test.NextHandler(dns.RcodeSuccess, nil),
+			qname:         "example.org",
+			metric:        "coredns_dns_response_rcode_count_total",
+			expectedValue: "4",
+		},
+	}
+
+	ctx := context.TODO()
+
+	for i, tc := range tests {
+		req := new(dns.Msg)
+		if tc.qtype == 0 {
+			tc.qtype = dns.TypeA
+		}
+		req.SetQuestion(dns.Fqdn(tc.qname), tc.qtype)
+		met.Next = tc.next
+
+		rec := dnsrecorder.New(&test.ResponseWriter{})
+		_, err := met.ServeDNS(ctx, rec, req)
+		if err != nil {
+			t.Fatalf("Test %d: Expected no error, but got %s", i, err)
+		}
+
+		result := mtest.Scrape(t, "http://"+ListenAddr+"/metrics")
+
+		if tc.expectedValue != "" {
+			got, _ := mtest.MetricValue(tc.metric, result)
+			if got != tc.expectedValue {
+				t.Errorf("Test %d: Expected value %s for metrics %s, but got %s", i, tc.expectedValue, tc.metric, got)
+			}
+		}
+	}
+}