plugin/metadata: add metadata plugin (#1894)

* plugin/metadata: add metadata plugin

* plugin/metadata: Add MD struct, refactor code, fix doc

* plugin/metadata: simplify metadata key

* plugin/metadata: improve setup_test

* Support of metadata by rewrite plugin. Move calculated variables to metadata.

* Move variables from metadata to pkg, add UTs, READMEs change, metadata small fixes

* Add client port validation to variables_test

* plugin/metadata: improve README

* plugin/metadata: rename methods

* plugin/metadata: Update Metadataer interface, update doc, cosmetic code changes

* plugin/metadata: move colllisions check to OnStartup(). Fix default variables metadataer.

* plugin/metadata: Fix comment for method setValue

* plugin/metadata: change variables order to fix linter warning

* plugin/metadata: rename Metadataer to Provider

plugin/pkg/variables/variables_test.go

@@ -0,0 +1,80 @@
+package variables
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/miekg/dns"
+)
+
+func TestGetValue(t *testing.T) {
+	// test.ResponseWriter has the following values:
+	// 		The remote will always be 10.240.0.1 and port 40212.
+	// 		The local address is always 127.0.0.1 and port 53.
+	tests := []struct {
+		varName       string
+		expectedValue []byte
+		shouldErr     bool
+	}{
+		{
+			queryName,
+			[]byte("example.com."),
+			false,
+		},
+		{
+			queryType,
+			[]byte{0x00, 0x01},
+			false,
+		},
+		{
+			clientIP,
+			[]byte{10, 240, 0, 1},
+			false,
+		},
+		{
+			clientPort,
+			[]byte{0x9D, 0x14},
+			false,
+		},
+		{
+			protocol,
+			[]byte("udp"),
+			false,
+		},
+		{
+			serverIP,
+			[]byte{127, 0, 0, 1},
+			false,
+		},
+		{
+			serverPort,
+			[]byte{0, 53},
+			false,
+		},
+		{
+			"wrong_var",
+			[]byte{},
+			true,
+		},
+	}
+
+	for i, tc := range tests {
+		m := new(dns.Msg)
+		m.SetQuestion("example.com.", dns.TypeA)
+		m.Question[0].Qclass = dns.ClassINET
+
+		value, err := GetValue(tc.varName, &test.ResponseWriter{}, m)
+
+		if tc.shouldErr && err == nil {
+			t.Errorf("Test %d: Expected error, but didn't recieve", i)
+		}
+		if !tc.shouldErr && err != nil {
+			t.Errorf("Test %d: Expected no error, but got error: %v", i, err.Error())
+		}
+
+		if !bytes.Equal(tc.expectedValue, value) {
+			t.Errorf("Test %d: Expected %v but got %v", i, tc.expectedValue, value)
+		}
+	}
+}