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_test.go

@@ -0,0 +1,42 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+func TestPrometheusParse(t *testing.T) {
+	tests := []struct {
+		input     string
+		shouldErr bool
+		addr      string
+	}{
+		// oks
+		{`prometheus`, false, "localhost:9153"},
+		{`prometheus localhost:53`, false, "localhost:53"},
+		// fails
+		{`prometheus {}`, true, ""},
+		{`prometheus /foo`, true, ""},
+		{`prometheus a b c`, true, ""},
+	}
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.input)
+		m, err := prometheusParse(c)
+		if test.shouldErr && err == nil {
+			t.Errorf("Test %v: Expected error but found nil", i)
+			continue
+		} else if !test.shouldErr && err != nil {
+			t.Errorf("Test %v: Expected no error but found error: %v", i, err)
+			continue
+		}
+
+		if test.shouldErr {
+			continue
+		}
+
+		if test.addr != m.Addr {
+			t.Errorf("Test %v: Expected address %s but found: %s", i, test.addr, m.Addr)
+		}
+	}
+}