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/handler.go

@@ -0,0 +1,34 @@
+package metrics
+
+import (
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics/vars"
+	"github.com/coredns/coredns/plugin/pkg/dnsrecorder"
+	"github.com/coredns/coredns/plugin/pkg/rcode"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// ServeDNS implements the Handler interface.
+func (m *Metrics) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	qname := state.QName()
+	zone := plugin.Zones(m.ZoneNames()).Matches(qname)
+	if zone == "" {
+		zone = "."
+	}
+
+	// Record response to get status code and size of the reply.
+	rw := dnsrecorder.New(w)
+	status, err := plugin.NextOrFailure(m.Name(), m.Next, ctx, rw, r)
+
+	vars.Report(state, zone, rcode.ToString(rw.Rcode), rw.Len, rw.Start)
+
+	return status, err
+}
+
+// Name implements the Handler interface.
+func (m *Metrics) Name() string { return "prometheus" }