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/trace/setup.go

@@ -0,0 +1,113 @@
+package trace
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("trace", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	t, err := traceParse(c)
+	if err != nil {
+		return plugin.Error("trace", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		t.Next = next
+		return t
+	})
+
+	c.OnStartup(t.OnStartup)
+
+	return nil
+}
+
+func traceParse(c *caddy.Controller) (*trace, error) {
+	var (
+		tr  = &trace{Endpoint: defEP, EndpointType: defEpType, every: 1, serviceName: defServiceName}
+		err error
+	)
+
+	cfg := dnsserver.GetConfig(c)
+	tr.ServiceEndpoint = cfg.ListenHost + ":" + cfg.Port
+	for c.Next() { // trace
+		var err error
+		args := c.RemainingArgs()
+		switch len(args) {
+		case 0:
+			tr.Endpoint, err = normalizeEndpoint(tr.EndpointType, defEP)
+		case 1:
+			tr.Endpoint, err = normalizeEndpoint(defEpType, args[0])
+		case 2:
+			tr.EndpointType = strings.ToLower(args[0])
+			tr.Endpoint, err = normalizeEndpoint(tr.EndpointType, args[1])
+		default:
+			err = c.ArgErr()
+		}
+		if err != nil {
+			return tr, err
+		}
+		for c.NextBlock() {
+			switch c.Val() {
+			case "every":
+				args := c.RemainingArgs()
+				if len(args) != 1 {
+					return nil, c.ArgErr()
+				}
+				tr.every, err = strconv.ParseUint(args[0], 10, 64)
+				if err != nil {
+					return nil, err
+				}
+			case "service":
+				args := c.RemainingArgs()
+				if len(args) != 1 {
+					return nil, c.ArgErr()
+				}
+				tr.serviceName = args[0]
+			case "client_server":
+				args := c.RemainingArgs()
+				if len(args) > 1 {
+					return nil, c.ArgErr()
+				}
+				tr.clientServer = true
+				if len(args) == 1 {
+					tr.clientServer, err = strconv.ParseBool(args[0])
+				}
+				if err != nil {
+					return nil, err
+				}
+			}
+		}
+	}
+	return tr, err
+}
+
+func normalizeEndpoint(epType, ep string) (string, error) {
+	switch epType {
+	case "zipkin":
+		if !strings.Contains(ep, "http") {
+			ep = "http://" + ep + "/api/v1/spans"
+		}
+		return ep, nil
+	default:
+		return "", fmt.Errorf("tracing endpoint type '%s' is not supported", epType)
+	}
+}
+
+const (
+	defEP          = "localhost:9411"
+	defEpType      = "zipkin"
+	defServiceName = "coredns"
+)