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/setup.go

@@ -0,0 +1,100 @@
+package metrics
+
+import (
+	"net"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("prometheus", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+
+	uniqAddr = addrs{a: make(map[string]int)}
+}
+
+func setup(c *caddy.Controller) error {
+	m, err := prometheusParse(c)
+	if err != nil {
+		return plugin.Error("prometheus", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		m.Next = next
+		return m
+	})
+
+	for a, v := range uniqAddr.a {
+		if v == todo {
+			// During restarts we will keep this handler running, BUG.
+			c.OncePerServerBlock(m.OnStartup)
+		}
+		uniqAddr.a[a] = done
+	}
+	c.OnFinalShutdown(m.OnShutdown)
+
+	return nil
+}
+
+func prometheusParse(c *caddy.Controller) (*Metrics, error) {
+	var (
+		met = &Metrics{Addr: addr, zoneMap: make(map[string]bool)}
+		err error
+	)
+
+	defer func() {
+		uniqAddr.SetAddress(met.Addr)
+	}()
+
+	for c.Next() {
+		if len(met.ZoneNames()) > 0 {
+			return met, c.Err("can only have one metrics module per server")
+		}
+
+		for _, z := range c.ServerBlockKeys {
+			met.AddZone(plugin.Host(z).Normalize())
+		}
+		args := c.RemainingArgs()
+
+		switch len(args) {
+		case 0:
+		case 1:
+			met.Addr = args[0]
+			_, _, e := net.SplitHostPort(met.Addr)
+			if e != nil {
+				return met, e
+			}
+		default:
+			return met, c.ArgErr()
+		}
+	}
+	return met, err
+}
+
+var uniqAddr addrs
+
+// Keep track on which addrs we listen, so we only start one listener.
+type addrs struct {
+	a map[string]int
+}
+
+func (a *addrs) SetAddress(addr string) {
+	// If already there and set to done, we've already started this listener.
+	if a.a[addr] == done {
+		return
+	}
+	a.a[addr] = todo
+}
+
+// Addr is the address the where the metrics are exported by default.
+const addr = "localhost:9153"
+
+const (
+	todo = 1
+	done = 2
+)