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.go

@@ -0,0 +1,101 @@
+// Package metrics implement a handler and plugin that provides Prometheus metrics.
+package metrics
+
+import (
+	"log"
+	"net"
+	"net/http"
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/metrics/vars"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+func init() {
+	prometheus.MustRegister(vars.RequestCount)
+	prometheus.MustRegister(vars.RequestDuration)
+	prometheus.MustRegister(vars.RequestSize)
+	prometheus.MustRegister(vars.RequestDo)
+	prometheus.MustRegister(vars.RequestType)
+
+	prometheus.MustRegister(vars.ResponseSize)
+	prometheus.MustRegister(vars.ResponseRcode)
+}
+
+// Metrics holds the prometheus configuration. The metrics' path is fixed to be /metrics
+type Metrics struct {
+	Next plugin.Handler
+	Addr string
+	ln   net.Listener
+	mux  *http.ServeMux
+
+	zoneNames []string
+	zoneMap   map[string]bool
+	zoneMu    sync.RWMutex
+}
+
+// AddZone adds zone z to m.
+func (m *Metrics) AddZone(z string) {
+	m.zoneMu.Lock()
+	m.zoneMap[z] = true
+	m.zoneNames = keys(m.zoneMap)
+	m.zoneMu.Unlock()
+}
+
+// RemoveZone remove zone z from m.
+func (m *Metrics) RemoveZone(z string) {
+	m.zoneMu.Lock()
+	delete(m.zoneMap, z)
+	m.zoneNames = keys(m.zoneMap)
+	m.zoneMu.Unlock()
+}
+
+// ZoneNames returns the zones of m.
+func (m *Metrics) ZoneNames() []string {
+	m.zoneMu.RLock()
+	s := m.zoneNames
+	m.zoneMu.RUnlock()
+	return s
+}
+
+// OnStartup sets up the metrics on startup.
+func (m *Metrics) OnStartup() error {
+	ln, err := net.Listen("tcp", m.Addr)
+	if err != nil {
+		log.Printf("[ERROR] Failed to start metrics handler: %s", err)
+		return err
+	}
+
+	m.ln = ln
+	ListenAddr = m.ln.Addr().String()
+
+	m.mux = http.NewServeMux()
+	m.mux.Handle("/metrics", prometheus.Handler())
+
+	go func() {
+		http.Serve(m.ln, m.mux)
+	}()
+	return nil
+}
+
+// OnShutdown tears down the metrics on shutdown and restart.
+func (m *Metrics) OnShutdown() error {
+	if m.ln != nil {
+		return m.ln.Close()
+	}
+	return nil
+}
+
+func keys(m map[string]bool) []string {
+	sx := []string{}
+	for k := range m {
+		sx = append(sx, k)
+	}
+	return sx
+}
+
+// ListenAddr is assigned the address of the prometheus listener. Its use is mainly in tests where
+// we listen on "localhost:0" and need to retrieve the actual address.
+var ListenAddr string